Stuck on her very own desert island in the year 5,002, Charley Pollard is delighted and amazed when she hears the familiar sound of the TARDIS materialising. She excitedly races inside the ship - only to stop in her tracks. She apologises to the person standing at the controls and explains that she was expecting to find someone else.

The sixth Doctor is surprised by Charley’s comment as he hadn’t realised dimensionally transcendental vehicles disguised as police boxes were quite so commonplace! Charley quickly realises what’s happened and claims she was simply over excited at being rescued. The Doctor says it makes a change from people saying how much bigger things are on the inside than the outside, but Charley says she was thinking how small it was. He introduces himself as the Doctor and Charley gives her name as Charlotte Smith, but her friends call her Charley. He tells her he’ll bear that in mind should they ever become friends. He says he picked up her distress signal and comments that Morse Code is a bit antiquated in this era, which makes him think she’s an old fashioned sort of lady.

He asks her how she ended up stranded in the year 5,002, but she says it’s a long and uneventful story and she doesn‘t want to bore him with the details. He asks where she’d like to be dropped off, but when she says she doesn’t want to go home, he tells her she’s not making much sense. She apologises and says it’s all been a lot to take in. The Doctor asks if Charlotte is her real name and he admits that he’s not entirely sure he trusts her. He’s surprised when the TARDIS starts to land and he suspects the ship must have found something of interest. On discovering they’ve landed in Manchester about half a million years back in time, he warns Charley that she’s actually standing inside a time machine. Charley feigns surprise, but not very convincingly. She’s about to suggest he switches on the scanner, but stops herself just in time as asks whether the ship has a means of seeing what’s outside. The Doctor opens the scanner and they see that they’ve landed inside a residential building. It’s late in the evening and the Doctor is worried they might be disturbing someone settling down to watch TV - but Charley spots a dead body on the floor. They decide to go outside and investigate, so Charley opens the door.

The Doctor rebukes Charley for being too impulsive and asks how she knew how to open the door, but she says she saw him close the door when she came in and just assumed it was the same lever. They examine the body and find he’s been strangled. Charley’s ‘old fashioned’ reaction to the murder makes the Doctor wonder exactly when she was born, but she sidesteps the question. She finds an open empty briefcase and guesses the killer must have wanted whatever was inside. The Doctor warns her not to jump to conclusions and asks whether she’s accustomed to stumbling across murder victims. The flat is cramped, dirty and neglected, and the electricity has been cut off, which makes the Doctor think no one’s lived here for quite a while.

Suddenly a cat leaps out and takes cover behind the dead body. The Doctor notes that there’s cat food and a litter tray in the kitchen, which means someone has been looking after it. With no sign of the murderer around, the Doctor decides to call the police, but when Charley picks up the telephone and tries to speak to the operator, he tells her to dial 999 instead. The phone doesn’t seem to be working, so she offers to go next door and see if they have a phone. The Doctor examines the body more closely and realises the man is wearing an expensive suit. Charley returns and tells him the front door has a sliding bolt on it, which means it was locked from the inside and the killer can’t have escaped through the window because they’re eight storeys up. After Charley lets herself out, the Doctor considers what he knows about her. She knows about phones so she must at least be from the 20th century, but everything else places her earlier rather than later.

Charley knocks on the nearest door and asks if she can use the phone urgently to call the police. The door opens and someone steps out and immediately knocks Charley unconscious, then drags her inside the room.

The Doctor is worried when Charley doesn’t return after twenty minutes and calls out for her, but there’s no response. A young woman appears in the corridor, wondering what all the fuss is about and sees the dead man through the open door. The Doctor tries to explain what’s happened, but she races back to her flat and slams the door shut behind her. The Doctor hopes she’s gone to call the police…

Elsewhere in the building, Maxine answer her mobile phone and assures the caller that there’s no reason for the police to come here as she‘s kept her nose clean so far. She confirms that the girl is still unconscious and she’s worried what will happen if she doesn’t recover, but she agrees that the money she’s being paid is good.

Down on ground level, a police car pulls up outside the building and PC Blackstock checks with HQ that it’s the eighth floor they need to go to. Detective Inspector Patricia Menzies also arrives and introduces herself as Blackstock only transferred here a few months ago. He tells her that DCI Turnbull is also on his way and the reports say there’s a bloke holed up inside Ackley House with a dead body. The reports say the suspect looks like a right nutter and is dressed in weird clothes, but they don’t know if he’s dangerous.

From inside the flat on the eighth floor, the Doctor hears the police downstairs calling out through a loud hailer. He opens the window and shouts a cheery hello down to the officers below. The police warn him to step away from the window and suddenly the door bursts open and officers swarm in, ordering him not to move. The Doctor calmly points out that he wasn’t moving and the door wasn’t even locked, but the officers order him to face the wall and spread his hands out. The woman from the corridor confirms that he’s the one she spoke to earlier and DI Menzies places the Doctor under arrest. As Menzies reads him his rights, PC Blackstock calls HQ and tells them the suspect has been subdued and restrained. Just then, he notices the police box in the corner and when the Doctor claims it belongs to him, Menzies orders it to be taped off so she can look at it later.

At the police station, DI Menzies begins questioning the Doctor. She introduces herself and asks whether his solicitor has arrived yet, but the Doctor says he doesn’t need one and despite the extremely serious charges, he’s confident his part in this can be cleared up soon. She starts the tape and gives the date and time as 1.05am on 29th February 2008. According to the custody record, the suspect is Dr John Smith of no fixed abode, but he has no form of identification on him and his date of birth is apparently unknown. The Doctor says he’s entirely innocent, but Menzies tells him the security cameras at Ackley House proves that the Doctor must have already been inside the flat when the victim entered and only one person was seen leaving the flat between then and when the police arrived. Menzies produces a photograph of Charley and the Doctor assures her she isn’t the killer and that he and Charley found the body together. In fact, he sent her to call the police and he’s worried about her, even though they’ve only just met. Menzies asks if he knows who the victim was as the body wasn’t carrying any ID either, but the Doctor says he doesn’t. She asks what he was doing in the flat as he isn’t the owner and the Doctor decides he has no choice but to tell her the truth. First, he asks her to go to the morgue and examine the victim’s body again more closely. He draws a diagram and asks her to check for an irregularity under the skin just under the shoulder blade.

Charley wakes up and finds herself on a bed with one hand chained to the post. She calls out, but no one responds. She spots a telephone and tries to make a call, but a woman enters and tells her not to waste her time as it hasn’t been connected for ages. The woman introduces herself as Maxine and tells Charley she has to stay here for a while. She offers to bring her some food and Charley is amazed to learn that you can get Indian food in Manchester now. Charley reels off a list of her favourite Indian dishes, but none of them seem to be on Maxine’s menu.

DI Menzies arrives at the police morgue and greets Dr Joseph Aldrich, noting that he looks unusually tired. She asks if there have been any developments, but he says the case is very straight forward - the victim was strangled and now he’s dead. Menzies says she loves to complicate things and asks if she can examine the victim’s back as there might be a clue there as to why he was killed. Aldrich laughs at her suggestion that he may have had something like a microchip implanted under the skin, but she asks him again and reluctantly he helps her turn the body over. While they’re doing that, Menzies reveals that the victim’s car was found parked outside the building and he’s been identified as Gregory Bailey. They’re trying to get in touch with his wife, but they’ve had no luck so far. Just below the dead man’s shoulder they find a small area of skin that looks like it’s overlapping, so Menzies asks Aldrich to cut it open. He peels the skin back to reveal a small object, but he says he can’t prise it out. Menzies takes over and manages to remove it, but just as she does so, the dead body begins to physically change shape. They’re both amazed and Aldrich says he’s never seen anything like it before…but Menzies thinks she knows someone who has.

Menzies storms back into the interrogation room and demands to know from the Doctor what’s going on. She tells him the body changed - his skin turned bright red and went like jelly, his whole build changed and his hair totally vanished. She’s still in shock and wonders if it’s all some sort of joke. She asks whether the Doctor set them up to look stupid by planting an alien body and she asks him about the implant. He tells her it’s a DNA patch that re-writes and stabilises someone’s DNA and in this case it changed the alien into a human, at least on the outside. He says he looked at the dead body while he was waiting for the police and spotted the tell-tale signs. He says there are two ways she can react to the news - she can either lock him away and pretend to herself that this is all nonsense, or she can let him help her solve the mystery. He tells her he knows about aliens and could probably reveal more if she’ll let him look at the body. She agrees, but warns him he’ll find himself down in the cells if he tries anything funny.

Maxine returns to Charley and complains that the lifts still aren’t working and she couldn’t get the delivery people to bring the food up the stairs. Charley asks again why she’s being held captive and Maxine tells her someone is paying her a lot of money to do it, but she doesn’t know why and she doesn’t care either. She says she’s waiting for a call and Charley will have to stay here until then. As she eats, Charley says it’s wonderful you can get Indian food at this time of night. When she was a young girl living in Hampshire, you’d have to go all the way to London to get a curry, but fortunately one of their footmen had served in India so he knew all the recipes. The phone rings and Maxine answers, telling the person on the other end that Charley is awake now.

Dr Aldrich is surprised when Menzies returns to the morgue bringing a friend with her, who she introduces as Dr John Smith. Aldrich assumes he’s a new member of the police staff, but Menzies explains that he’s actually the suspect and he’s asked to look at the body. Aldrich is outraged, but the Doctor cuts him short and immediately identifies the alien body as a Shinx, one of the inhabitants of the planet Shinus. They’re economically a very successful species and generally speaking quite low-key and although he doesn’t know a lot about them, he’s surprised to see one here. Aldrich protests, so Menzies asks him to leave and says she‘ll put the body into storage once they’ve finished. Once they’re alone, the Doctor tells Menzies that he thinks the murderer may have been wasting his time. He’s noticed some discolouring around the nostrils and he believes the alien was terminally ill, so the killer could probably just have waited and saved himself the trouble. Menzies ponders whether somebody needed him out of the way quickly. The Doctor mentions the empty briefcase and wonders what could have been inside it that was worth killing someone for.

Maxine hangs up and Charley asks what her new instructions are. Maxine says she’ll be allowed to go as soon as she tells them what happened in the room where the man was murdered. Charley tells her all she knows, but Maxine says that’s not good enough. Someone has told Maxine that they saw Charley coming out of the room, so they know she must have been inside the whole time, but Charley insists that neither she nor the Doctor know what happened. Maxine thinks Charley won’t be leaving anytime soon…

Menzies asks to have a word with her boss and tells him there’s more to this case than they thought. She says the whole thing is odd and asks if Aldrich told him what they discovered about the body. He obviously didn’t so she takes Turnbull into the mortuary.

Maxine tells Charley she‘s popping out to get some cigarettes and advises her to spend some time thinking about what she knows. The telephone next to Charley’s bed rings unexpectedly, so Charley answers it cautiously. A man’s voice on the other end introduces himself as Sam and pleads with her not to hang up. He says he’s been calling people all over the building, but everyone hangs up because they think he’s messing about. He asks for her help, but then she hears Maxine returning, so she hangs up. Maxine says she forgot to take her lighter and leaves again. As soon as she’s gone, Charley picks up the phone, but there’s no one there.

Menzies and Turnbull join Dr Aldrich and he takes them to the drawer where Gregory Bailey’s body is in storage, but when Turnbull sees the alien body inside he accuses them of playing a joke on him. If it was anyone else, he’d assume they just had an overactive imagination, but he knows Menzies too well. He’s about to tear a strip off them when there’s a knock at the door and Gregory Bailey himself pops his head round and asks if they’re looking for him! Everyone is shocked until he explains that he woke up about twenty minutes ago and found himself under a sheet. Turnbull asks Menzies to confirm that Bailey is the supposed murder victim and when she says he is, Turnbull diagnoses him as being very much alive! Aldrich is deeply embarrassed and confused, but Bailey explains that he must have misjudged his own medication. Menzies asks whether he’s sure he hasn’t been assaulted, but Bailey swears that he’s fine and apologises for causing any trouble. He asks if he can go home and although Menzies would like to question him further, Turnbull overrules her and allows him to leave. Then Turnbull demands that Menzies and Aldrich explain how this happened as it’s left them wide open to a future lawsuit. It’s just lucky Aldrich hadn’t already started the autopsy and sliced the poor man open! He doesn’t even want to ask what the rubber monster is doing on the slab and he suggests they both take some time off. Menzies assures him she’s fine, but he insists - although he adds that he’d like to see them both tomorrow afternoon so they can go over this fiasco. He also tells them to let the Doctor go as it seems they haven’t even got a murder to charge him with any more.

It’s 5.00am and Charley is asleep when the phone rings again. Drowsily, she answers it and recognises the voice at the end as Sam. He asks her why she hung up on him and she explains that someone came into the room and she didn’t want her to know that she was talking to him. Sam is surprised to discover that it’s night time and says he can’t tell because it’s dark where he is and he can‘t see anything. He tells her he’s trapped in the basement, but nobody else in the building will help him. He can’t remember what happened and all he knows is that someone locked him in here and he can’t get out. Unfortunately Charley has similar problems of her own and she wonders if the same people are responsible for locking them both up. She tells Sam she’s planning to escape as she needs to find her friend the Doctor and explain everything to him as she feels she’s let him down. She tells Sam she’ll wait until her captor comes back and asks him to be patient until then and try not to panic.

Menzies is furious to learn from PC Blackstock that they’ve lost the Doctor. He argues that she said he was no longer a suspect, but she says that’s not the point. The officer only turned his back on the Doctor for a second, but he’d gone by the time he looked round. Menzies is about to check the internal CCTV footage when the phone rings and it’s the Doctor on the end of the line. She asks him how he got her number and he explains that DCI Turnbull gave it to him and suggested he keep in touch with her. Of course, he was taking on the appearance of the late Gregory Bailey at the time! Menzies realises the man they thought was the miraculously recovered murder victim was actually the Doctor in disguise. He admits that he pocketed the DNA patch they removed from the body and he used it on himself. He’s ringing her firstly to apologise, but also to remind her that a man is still dead. He wants to know what really happened and he assumes she’d like to get to the bottom of it too. Menzies says the investigation has been formally closed but when he offers to show her how he managed to get inside the room after the murder happened, she agrees to meet him back at the crime scene.

The Doctor meets up with Menzies and apologises for being late. They enter the flat on the eighth floor and cut through the police tape around the TARDIS. The Doctor tells her to watch carefully and he promises not to do a runner. He says he’ll aim for the corner just across from the shops about five minutes down the road, then he steps inside the TARDIS and it dematerialises. Menzies is astonished…

Charley answers the phone again and Sam asks her if she heard the sound and she says she did. She tells him it was her friend the Doctor leaving in the TARDIS. She realises he must have gone and left her here. She accepts that he’s not the man she knew, at least not yet, and is resigned to the fact that she’s going to have to look after herself from now on. Sam asks when she’s coming down to help him and she promises to come soon, but asks him not to call again in case her captor hears him.

Menzies leaves the building and goes over to the TARDIS, which is now standing exactly where the Doctor predicted it would be - just across from the shops about five minutes down the road. When the Doctor comes out and greets her, she decides to take what she’s seen at face value. She now knows how he got inside the building, but if he’s really not the murderer then that makes Menzies’ job significantly harder. The Doctor plans to investigate and invites her to help, but she says it’ll have to be in an unofficial capacity as she’s been ordered to go on leave. She notices his hair is wet and he says he’s had a shower and ironed some shirts since he last saw her a few minutes ago. He’s also had time to read a document on Shinx anatomy, which confirms his earlier diagnosis that the alien pretending to be Gregory Bailey would have been dead within days from a disease of the respiratory system. It may even have been accelerated due to him having to adjust to the atmosphere of this planet. Menzies wants to know why he was in a shabby old flat in Manchester and the Doctor suspects he was doing one of two things - he was either assessing the planet’s economic potential ahead of making official first contact, or he was covertly trading with people here whilst avoiding making first contact. Menzies suggests he might have been finding a wife, which surprises the Doctor. She reveals that Gregory was married and thinks this might be the intergalactic version of getting a bride on the internet. Menzies decides to take the Doctor to the “posh end of town” so he can meet Mr Bailey’s wife.

At his surgery, Dr Aldrich asks his secretary to send in the next patient. He becomes concerned when a man named Slater enters and tells him he has a pain in his chest which he thinks is due to the fact that he’s worried after hearing about Aldrich’s trip to the police station last night. Aldrich assures him he was just fulfilling his public service duty and says it’s not in his interests to tell the police anything and he knows better than to stand in Slater’s way. In any case, the investigation is over and they’ve dropped the case. Slater is pleased and makes a call and cancels his plans to have Dr Aldrich “dealt with”. Slater says he also has a separate medical issue for Aldrich as his prescription needs renewing…

The Doctor and DCI Menzies visit Antonia Bailey and tell her they were hoping to speak to her husband in connection with an investigation. Antonia says he works for an environmental charity but he’s on some business that will keep him out of town overnight. The Doctor asks if she’s familiar with a block of flats called Ackley House out near Gorton, but she says it doesn’t ring a bell. He tells her the crime they’re investigating took place in a flat there and there’s evidence to suggest her husband might have owned it. Antonia says that’s news to her but she admits that she and her husband lead somewhat separate lives and they don’t tend to come home and chat about their work over dinner. They ask if they can check some of her husband’s paper and Antonia agrees.

Maxine brings her prisoner some breakfast, but Charley’s never seen chocolate flavour rice pops before. Charley notices something is wrong and Maxine admits that she didn’t sleep very well. Charley asks to be released from her handcuffs, but Maxine says she hasn’t got the key. Maxine is feeling bad about what’s happening, but the money she’s been offered is too great to turn down. She really needs it because the building has been condemned and it looks like she’s going to get thrown out soon and she has nowhere else to go.

Once they’re alone, Menzies asks the Doctor what he makes of Gregory Bailey’s wife. He flippantly says she’s an alien too and her body language has given her away. He can’t tell for sure if she’s the same species, but it seems likely. Menzies wonders whether Antonia has suspected they know the truth, but the Doctor doesn‘t think so. He doesn’t think they’ll find anything useful here as the main objective of the aliens is to give the appearance of normality. Menzies agrees that the house looks like it’s come straight from a catalogue.

The phone next to Charley rings again and Sam asks her if it’s alright to talk. Fortunately they’re in luck as Maxine has just gone out for five minutes. Sam asks if she thinks she’ll be able to escape, but Charley says the keys to the handcuffs aren’t kept in the flat. She can’t think of any way out and feels bad about not being able to help Sam - but as she turns her attention back to the handcuffs, she’s amazed to see that they’re coming apart and the links are breaking off from rust. She knew it was filthy in this place, but she didn’t think things were that bad. She manages to free herself and Sam asks her to come and help him in the basement. Charley says she’ll have to wait for Maxine to come back as the front door is locked too, but she promises to come for Sam as soon as she can.

Menzies tells Antonia that she’s been very helpful, but before they go the Doctor has one last question. He asks whether her husband had been scaling back his work at all recently, considering his poor health. Antonia is surprised by this and is even more shocked when the Doctor reveals that Gregory is terminally ill. Menzies suggests her husband may have been keeping the news from her and the Doctor says they should speak to the family doctor to establish the facts. Antonia reveals that his name is Joseph Aldrich…

Maxine returns to the flat with some more food for Charley and finds her apparently motionless on the bed. As she moves closer, Charley lashes out and holds her in a head lock. She demands Maxine hand over the front door keys and her telephone, then she orders her to stay here. Maxine asks why she didn’t just tell the people what they wanted to know, but Charley says she’s been telling the truth all along and she has no idea what they’re talking about. Suddenly Maxine tries to fight back, so Charley has no choice but to knock her unconscious. She then leaves and locks the door behind her.

Dr Aldrich is surprised when DCI Menzies and the Doctor arrive at his surgery. Menzies says she has some questions about what happened last night, but Aldrich says he’d rather pretend last night never happened. He tells them he hasn’t got time to speak as he’s expecting another patient, but the Doctor says he’s already spoken to the man in the waiting room and diagnosed him personally. Aldrich is outraged, but Menzies warns him that before he tries to claim the moral high ground he might want to explain why he lied to her yesterday about not knowing Gregory Bailey as they now know he was one of his patients. Aldrich says he was promised the connection between them wouldn’t be traced and he didn’t want anyone to know he was treating Bailey. The Doctor says his wife Antonia was very helpful, although she didn’t want them to know she wasn’t human either. The Doctor asks Aldrich where he picked up the knowledge to work on aliens and he says it all started when he had a run-in with a species called the Rolteans, a rotund and heavily armoured species who don‘t speak human languages very clearly. Aldrich says two of them crashed on Saddleworth Moor in 1991. One of them was badly hurt and the other one kidnapped Aldrich, but he found a way to patch up the pilot and they were grateful and rewarding. Before long, they were recommending him to others and he started acquiring knowledge of other alien species. As time went by, he started getting better at spotting aliens who were hiding among them. He started working for the police so that he could keep a monopoly on this area by removing any evidence of aliens who turned up dead. Menzies promises to get him fired, but Aldrich says that’s the least of his worries.

Charley is making her way down from the staircase of Ackley House when the phone she took from Maxine starts to ring. She answers it and it’s Sam. She asks him how he got this number, but he just tells her to hurry down as quickly as she can. Suddenly Charley spots a young woman standing on a ledge, apparently threatening to jump off the building. She tries to persuade the woman to step back for her own safety, but the woman warns Charley to keep away…

As DCI Menzies drives them back to the police station, the Doctor asks Aldrich who else he was treating that wasn’t from around these parts, but he refuses to answer on the grounds of confidentiality. The Doctor warns him that he knows people he could report him to, but he also assures Aldrich the police will protect him and he’ll make sure the aliens are treated justly.

Charley tries to convince the woman that things can’t be that bad, but the woman points down to the ground level where the dead body of her boyfriend can be seen. She explains that he killed himself and it was her fault because they had an argument. Charley tells her not to blame herself, but this makes the woman become hostile and she turns the argument around to blame Charley. She climbs down and grabs hold of Charley who tries to assure her she was just being sympathetic, but the two of them start to struggle and the woman swears she won’t let Charley get away with what she’s done…

The Doctor struggles for breath, but eventually he manages to summon the strength to call for some chocolate. Aldrich points Menzies to a vending machine in the canteen and the Doctor pleads with her to hurry.

Charley tries to assure the angry young woman that she didn’t come here to hurt anyone, but the woman refuses to let her go. Charley manages to break free and rushes to the lift, pressing the button frantically. The woman tells her not to waste her time as the lift hasn’t worked in ages, but then surprisingly the lift doors open. Charley leaps in and presses the button to go down, waving at the woman as the doors close in her face.

Menzies returns to the Doctor with a hot chocolate and helps him to drink as much as he can. As he starts to recover, the Doctor reveals that some soluble aspirin had been put in the tea and it could easily have killed him. The chocolate was a ideal source of simple triglycerides which was easy for him to absorb and which combated the effect of the aspirin. Aldrich thinks he’s talking nonsense unless his physiology is completely different. It slowly dawns on him that the Doctor is an alien too, but more importantly the Doctor realises someone must have known he would have that reaction. Aldrich recalls that once before someone had broken into his office overnight and took all his records relating to the Shinx. He has no idea how they did it, but he promises to tell the Doctor and Maxine everything they want to know before they get to him first!

Charley is confused by a ringing noise inside the lift and realises it’s another phone. She lifts the receiver and is delighted to hear Sam on the other end. She tells him the lift has become stuck and although she tried to call for help, the phone she took from Maxine won’t work inside. He explains that she probably can’t get a signal and she takes his word for it. She wonders how Sam knew she was in the lift, but he tells her it was just a good guess. Moments later the lift starts working again and Charley hopes the angry woman isn’t at the bottom when she gets there…

Aldrich tells Menzies about the aliens he’s worked with. He mentions a man called Robert Slater who employs people called Dan Burgess, Steve Brooks and James Carver. He doesn’t know much about Gregory Bailey except that he operated an embassy for other Shinx on Earth and people would go to him if they had any problems. The Doctor says this isn’t unusual for planets who haven’t yet made first contact and he wonders if Bailey was killed because he refused to help somebody. Menzies decides it’s time to speak to some of the people Aldrich mentioned, but he asks them not to reveal his name as he doesn’t want any of this getting back to him.

Emerging from the lift on the ground floor, Charley calls for the police and an ambulance and says she’s in a building called Ackley House somewhere in Manchester. She tells them there’s been a suicide and also that she’s been attacked, plus there was a murder here earlier (just in case they didn’t already know) and there‘s another girl who‘s been paid to keep her prisoner. She hangs up and decides to go and look for Sam.

Menzies decides to visit James Carver first because the other three work together and she doesn’t fancy dealing with them all at the same time. The Doctor doesn’t seem convinced, but when she reminds him that she’s still in charge of the investigation, he’s reluctantly prepared to let her do things her way. They arrive at their destination just in time to see some men load a heavy bin bag into the boot of a car. Menzies introduces herself and asks if she can see what they’ve just put in the boot. The men panic and knock her aside, then jump into the car and race off at speed. The Doctor agrees that this is very suspicious behaviour, so they both return to the police car and give pursuit.

Charley makes her way down to the basement and is surprised to find the door isn’t even locked. The phone rings again and Sam asks where she is. She tells him it’s very dark down here - not a normal dark, but a really deep dark like being inside a cave. He asks her to turn the light on, but it doesn’t work so she tells him to guide her by his voice. He thinks she should already be able to hear him now, but she can’t so she switches off the phone and tells him to call out loudly. There’s nothing, and then the phone rings again. Sam asks whether she could hear him because he could hear her perfectly well. He calls out again, but she can only hear him when he’s on the phone. She finds the lighter that she took from Maxine and flicks it on. Amazingly, she can’t see any flame coming from it yet she can feel its heat. She realises there’s something very wrong here - it’s as if there’s something down here that’s actually eating the light. She begins to panic and heads back for the door, ignoring Sam’s desperate calls for her not to leave him behind in the dark. She races back up to ground level and bumps straight into the woman who slammed the door in her face shortly after they first arrived here. The woman produces a gun and orders her not to move, then demands to know how she got out. She says she always knew Maxine would be useless and Charley realises she’s the one who was paying to keep her prisoner. The woman orders her to start making her way back upstairs.

The Doctor urges Menzies to drive faster as they don’t want to loose the men. He assumes they’re both thinking the same thing about what might be inside the bag they loaded, although they both hope they’re wrong! As they turn into Claremont Road, the car they’re chasing tries to overtake someone on a dangerous bend and inevitably the car crashes into oncoming traffic. The Doctor leaps out to check that everyone’s alright while Menzies calls for an ambulance. Fortunately the driver of the other vehicle is only shaken, but the two men they were chasing look like they could do with some attention. The Doctor and Menzies check in the boot and unwrap the bin liner to reveal a dead body - almost certainly that of James Carver. Menzies contacts HQ again and asks Dr Aldrich to join them.

The woman, who introduces herself as Antonia, orders Charley to go with her to flat 83, just a few doors down from the room where the murder of Gregory Bailey took place. Charley tells her she’s already called the police, but Antonia thinks they‘ll have their hands full with the bodies spread out over the plaza downstairs. She asks Charley if she knows a man named Carver as he’s late for their appointment. She then reveals that the murder victim was her husband and although this isn’t exactly the place she’d have chosen for him to meet his end, she’s not sure where else she would have chosen…

Aldrich joins the others at the scene of the road accident and confirms that the dead body is Carver. The Doctor wonders what the man might have done to deserve this, but Aldrich only knows that he was writing a book. Menzies asks Aldrich to accompany the body back to the station, then she tells the Doctor the two men in the front aren’t making much sense and it probably isn’t just because they have head injuries. There’s a connection between this case and the Bailey murder because both victims were strangled, but why would they take this body away? Menzies has also learned there have been two further deaths back at Ackley House. Both were suicides although one jumped from the balcony and the other took pills. Officers are on their way now in response to a call they got from Charlotte Pollard. The Doctor doesn’t recognise the name, but he spots the connection with his new travelling companion and thinks the coincidence is too great. He decides to go to the scene himself as everything seems to centre around that building…

DCI Turnbull is surprised when he turns up at the scene and finds DI Menzies in charge, especially as she’s supposed to be on leave, but she assures him she was just following a hunch when she saw the men acting suspiciously. He’s even more amazed to see the Doctor with her, but she tells him he just wanted to pick up some personal possessions from the scene of his arrest the night before. Turnbull doesn’t have time to deal with them and orders Menzies to go home as he doesn’t want her involved in this or any other case right now. As Turnbull leaves, the Doctor suggests to Menzies that she speaks to Robert Slater while he goes back to Ackley House to see what’s going on there. Menzies is worried about meeting an alien, but the Doctor tells her she’ll be alright if she doesn’t let on that she knows. The Doctor says he’ll catch a bus to the flats but Menzies tells him to take one of her spare phones with him just in case.

Antonia tells Charley she still hasn’t set foot inside her husband’s flat even though she was keeping an eye on it. She promises to let Charley go if she’ll tell her what happened in there, but Charley says she’s already explained that when they found him he was already dead and she doesn’t know who killed him. Antonia reveals that she has no interest in who killed him…

Menzies brings Slater to the police station to “help them with their enquiries”, but she assures him this is just an informal chat as they need to find out more about Gregory Bailey, who’s gone missing. Slater promises to help in any way he can, but adds that he doesn’t actually know Bailey all that well. Slater says he’s researching into renewable energy sources and he needed to secure significant funding. He admits that he actually came into contact with Bailey through his wife Antonia after they met at a fundraiser and she suggested her husband might be able to put some capital into the business.

The Doctor calls Menzies and says he’s arrived at the block of flats and everything seems eerily quiet. He decides to take a look around. In the flat above, Antonia looks through the window and sees him walking around downstairs. Charley suggests they invite him up too and Antonia realises she’s not taking her situation seriously. She wonders if Charley doubts her ability to kill her, so she tells her justice on this planet means nothing to her and she can soon be far, far away and beyond the reach of the authorities here. Antonia says she didn’t plan to tell her more than was necessary, but now she’s changed her mind. She reveals that she knows who killed her husband…because she planned it with him!

Menzies tells Slater that Mr and Mrs Bailey weren’t particularly close and he responds in a way that she says confirms certain suspicions she had. She thanks him and says he’s been extremely helpful. Then the phone rings again and Aldrich asks her to come back straight away as there have been some interesting developments. Slater says he’ll be happy to be of further assistance, then he leaves.

Maxine’s doorbell rings and she asks who it is. The voice of the Doctor calls back asking if he can come in and speak to her. She tells him the door is locked and she can’t get out, so he picks the lock with a paperclip. She thanks him and says she was starting to think she’d never get out of here. He tells her Charlotte had called to say she was being held prisoner in this flat and Maxine confesses straight away, claiming that another woman kidnapped Charley, then paid her to keep her locked up until she revealed what happened in the flat where the man died. The Doctor comments that it’s very dark in here and he asks Maxine to turn the lights on - but she says the lights are already on!

Antonia explains that she and her partner Sam followed her husband out here, but they didn’t think they’d need to cover their crime up because they’d be gone soon. When she found out Gregory was coming here almost every day without telling anyone, she couldn’t believe how perfect it was. Sam broke in one day and waited for Gregory while she hid outside and signalled to Sam when she saw her husband arrive. She watched both of them go in, then waited and waited, but neither of them came out again. She says she was worried Gregory had made Sam a better offer - but then Charley came out. She demands to know what happened in there and where Sam is, but even when Antonia points the gun straight at her head, Charley insists that she doesn’t know anything. Antonia gives her a count of three before she fires, but just as she’s about to reach zero, the phone rings unexpectedly.

Maxine’s phone rings, which confuses her as she was cut off for not paying the bill for over a year. She even checked it earlier this afternoon when she got locked in here, just in case. The Doctor decides to answer the phone himself, but she warns him off and tells him she’s heard stories about people seeing ghosts. Officially it was all put down to carbon monoxide leaks, but she never believed that. The Doctor tells her to be quiet and they can hear phones going off in every room all over the building.

Charley asks Antonia why she doesn’t pick up the phone, but she says no one is supposed to know she’s here. Charley decides to answer it herself and finds Sam on the other end. She hands the phone to Antonia and Sam tells her she mustn’t kill Charley as she tried to help him. Sam seems confused as says something strange is happening, then he suddenly remembers who Antonia is and he confirms that he killed Gregory. Just then the door bursts open and the Doctor comes in and greets Charlotte warmly. He also says he was hoping to find Antonia here.

Menzies joins Aldrich in the mortuary at the police station and asks him what he’s got. He shows her a key which he says he retrieved from inside Carver’s stomach. He also found a card in his wallet which suggests the key belongs to a locker in a private gym. Aldrich casually asks what Slater told her, but she says it’s none of his business. He warns her to be careful with Slater and he finally reveals that Slater came to him once with one of his own people who needed treatment. She was literally falling to pieces, and although Slater refused to say how it had happened, Aldrich knows it was something he’d done and it wasn’t an accident.

The Doctor urges Sam to calm down and tells him he’s putting everyone in the building in danger. Sam refuses to listen and then they hear the sound of the door to the flat slamming shut. Charley tries it, but finds it’s jammed. The Doctor tells Sam to trust him and let them go, but Sam says that if he can’t leave, then none of them will either. Charley speaks to Sam in a comforting voice and reminds him that he helped her earlier. She now knows it was he who made the phones and the lifts work and he who released her from the handcuffs. But she tells him everyone in Ackley House is living in fear and anger, and it’s him that’s causing it. Sam says he’s not doing it deliberately, but he admits that he feels guilty about the murder and something just flipped in his head when he saw the life go out of his victim. Sam tells him he has to learn to live with what he did and there are innocent people killing themselves because of him. The longer it goes on, the more deaths he will cause. Sam says he doesn’t care anymore, so Antonia takes over and reminds him that he’s not a bad person. She blames herself for what happened and reveals that she lied to Sam about what her husband was doing. She made up everything she’d said about Greg being involved in abductions and it’s all because they don’t have divorces where she comes from. She’d drifted apart from Gregory because he was neglectful and obsessed with his projects, but he wasn’t a bad man either and he loved this planet. Sam asks why she didn’t just leave him, but Antonia says she would never have been allowed to go home again. She also admits that because nobody wants to work the high-risk planets like Earth, there’s an incentive package from their government which gives a high payment for any accident or death that occurs while at work. The Doctor is outraged to learn that this has all been about money!

Dr Aldrich is surprised when Slater arrives at the police station, but Slater tells him he can get in anywhere. Perhaps if he’d known that, Aldrich might not have been so quick to talk to Menzies and the Doctor. Aldrich insists it was Antonia who revealed that he’d been helping them and he blames Slater for being so secretive in the first place. Slater asks what he found inside the body of Carver and is told it was a key to a gymnasium locker. Slater wonders if the locker contains the second of the two cases that Gregory Bailey stole and he hopes Menzies won’t be tempted to open it as the consequences would be very unfortunate…

Menzies arrives at the gymnasium and receives the customary wolf-whistles from the half dressed men in the changing room. She produces her police ID card and gives the men ten seconds to vacate the area. Once she’s alone, she finds the correct locker and opens it, praying that it won’t just contain someone’s stinking trainers. As she pulls out a case, the door to the changing room opens and one of Slater’s thugs, a man named Ron, enters. He pulls out a gun and orders Menzies to hand over the case.

Antonia’s assurances to Sam seem to be calming him down, so the Doctor thinks it’s time to get the police out here and tell them what they know. Charley says she called the police ages ago, but the Doctor says Sam has been generating a sense of foreboding which is keeping people away. Unfortunately, while investigating the murder of Bailey, the Doctor stumbled across the murder of Carver and he doesn’t think Antonia and Sam were responsible for that. There’s also a lot of powerful alien technology around which needs to be collected and removed, but Antonia says she can help with that as her husband was doing business with a man named Slater. During their discussion, the Doctor remembers that Charley used the name ’Pollard’ when she called the police, but when they first met she told him her name was ’Smith’. Charley claims that it’s her maiden name and she still gets confused, but the Doctor knows she’s lying. He realises Pollard is her real name and tells her they need to have a talk about all this later.

Slater tells Ron to bring Menzies to his office, but he adds that under no circumstances is he to open the briefcase. Aldrich wonders what Slater’s plans for him are and warns him he can’t kill him in the middle of a police station. Slater assures him he can and says it’s just a coincidence that everyone in the building happened to be looking the other way when he entered and that they will be again when he leaves. He also says there’s a temporary fault with the internals security cameras. He apologises to Aldrich and pulls out a weapon…

Antonia says Sam is ready to talk and the Doctor hopes this means he’s also ready to let them all out. Sam asks the Doctor if he understands what’s happened to him, but the Doctor has to admit that he’s never seen anything quite like this before and he’s not entirely sure it’ll be reversible. Sam says that’s not good enough but the Doctor reminds him it only happened because he killed someone. Sam says he won’t let them go until they sort the problem out, then the phone goes dead. The Doctor turns to Antonia and asks if she has Slater’s phone number…

In his car, Slater unexpectedly receives a call from the Doctor and tells him it’s lovely to speak to him at last. The Doctor cuts him short and says they don’t have time for fake pleasantries. He asks Slater if he’s aware of what’s been happening at Ackley House and then asks if he was the one who engineered the device inside the empty briefcase. Slater corrects him and says it wasn’t a device, but a form of radiation that could cause a change of state so that anything it came into contact with was no longer a solid, a liquid or a gas. It was designed to enable a person to permeate their own environment, but it still needs refining as all their previous test subjects simply lost the ability to communicate. Slater says he’d welcome the Doctor’s input, but the Doctor wants to know if the effect can be reversed. Slater says he’s on his way to do that right now as he never intended for this to happen at all. He hangs up as Menzies complains that on this planet they have rules about beating and kidnapping police officers. Slater is intrigued to discover that she knows he’s not human, especially when she claims the Doctor knows all about them. Menzies asks him if he’s responsible for killing Carver and trying to kill the Doctor and he admits that he is. She asks him what’s inside the briefcase he’s carrying and he tells her it’s an antidote.

Through the windows of the building, the Doctor sees Slater’s car pull up outside. Antonia picks up the phone and tells Sam that she knows Slater doesn’t like her and she’d like permission to be let out before he gets there. Sam refuses and points out that Antonia lied to him and everything that’s happened is her fault. She begs him to be reasonable, but he hangs up. Slater presses the buzzer and the doors of Ackley House open and allow him to enter with Menzies and Ron. He asks Ron to take Menzies and the briefcase down to the basement.

The Doctor contacts Sam again and tells him he’s not convinced that Slater means to help them and he recommends that Sam keep quiet for the time being. He suspects Slater has underestimated Sam and it’ll be useful to see what his intentions are before they show their hand. Slater enters the room with a gun and orders everyone to keep their hands where he can see them. He addresses Antonia by the name Lyria and offers his condolences on the passing of her husband Kord, even though he knows full well that she arranged for him to be killed. Antonia warns the others that Slater, who’s real name is Vorath, always planned to take this world by stealth, but she doesn’t think he’d ever get their government to back his ideas as they’re too expensive, too risky and ethically dodgy. Slater tells her she’s out of touch as he has a lot of support back home and the Doctor realises he allowed Antonia to kill her husband, an ambassador on an alien planet, in order to promote the idea that Earth is a hostile world. He must also have allowed Bailey to steal and test the technology, no doubt because he’d run out of willing subjects. He would have known Bailey was dying and was keen to use the technology to shake off his old body and move on. Slater admits that he got Juros, the man they knew as Carver, to help him, but the Doctor has realised that Juros was never on Slater’s side and tried to hide the other briefcase from him, so Slater had him killed. The Doctor wonders whether the aliens intend to invade Earth by inhabiting its architecture and Slater confirms that this is a far more effective method than just storming the planet. In fact, they’ve been working on ways to influence Earth’s environment for years, by manipulating the mood of the population. Anyone who doesn’t agree with them will end up doing away with themselves and no one will be any the wiser.

Slater says he’s come here to help liberate Sam using the second case that he retrieved from Carver. The Doctor asks if it will change Sam back, but Slater says it will simply destroy the building and reduce everything inside to dust particles too small for him to inhabit. The Doctor is upset to learn that the process isn’t reversible and realises Slater is only concerned that Sam might soon learn how to make proper use of his new capacities and start working against him. Slater adds that he plans to lock everyone inside the building and remove all his problems in one go. The Doctor reminds him that there’s still a police investigation underway, but Slater doesn’t care as he’s returning to their homeworld anyway. He invites Antonia to come with him and says it would be helpful if she corroborated his story about what happened to her husband when he’s questioned by the authorities back home. At first she refuses, but he says that although he can cope perfectly well without her, her story will make his case sound more compelling. Eventually she agrees to join forces with him, much to Charley’s anger.

Elsewhere in the building, Maxine’s phone rings and when she answers it, she finds Sam on the other end. He introduces himself and says he needs her help…

Charley discreetly asks the Doctor whether they have a plan. He assures her he always has a plan, although on this occasion it’s largely reliant on the actions of others, in particular Sam. Charley thinks Sam’s been very quiet recently, but the Doctor says he’s being very clever. Slater joins them and announces that the device is now primed to go off in ten minutes, which will be just enough time for him and Antonia to get clear. They leave and a few minutes later, the door opens again and Maxine pops her head around to ask if it’s all clear. She reveals that Sam called her and asked her to help them. Just then, Sam phones and asks Charley if the Doctor will be able to stop the bomb in the basement from going off. The Doctor thinks he probably can, but they’ll need to wait until Slater is out of the way. Charley has an idea and asks Sam to bring the lift up to this floor…

As Slater and Antonia walk down the stairs towards ground level, he tells her he’s sorry about what’s going to happen, but she doesn’t believe him and says he never approved of her relationship with Sam. Suddenly the lift doors open to reveal Charley inside. She reaches out and pulls them both inside just as the doors close again and the lift starts to descend. Slater struggles to force open the doors, but Antonia knows he’s wasting his time. The internal phone buzzes and Slater is stunned to hear the voice of Sam Chapman on the other end. Sam tells him they’ve met very briefly a couple of times, but knows Slater probably won’t remember him. Slater asks where he is and Sam says he’s all around him. The revelation hits Slater like a bolt. Sam says he knows this place as he grew up only a few streets away and he even went out with a girl who lived in this building. Slater deduces that because Sam knows the environment well, the adjustment has been a lot quicker than he anticipated. Slater admits that he plans to destroy the building and says that he has no choice, but he adds that Sam can still save his beloved Antonia - if he’s prepared to let him go too. Sam says he doesn’t care if they all die together and he has no love for Antonia any more after discovering that she lied to him. Slater tries a different approach and threatens to shoot both Antonia and Charley, but Sam tells him it won’t get him out of here any quicker…in fact, it won’t get him out of here at all.

Following Slater’s instructions, Ron has taken Menzies down to the basement. She asks him how long he’s known that his boss is an alien, but he ignores her question and orders her to sit down and shut up. Just then, Ron is distracted by the arrival of the Doctor and Maxine, giving Menzies the opportunity to knock the thug out. She tells the Doctor there’s a bomb somewhere in the basement, so the Doctor asks Maxine to keep watch outside while he and Menzies search for it.

Slater points out that the bomb has already been set and he can’t stop it while he’s trapped inside the lift. Sam says he’ll consider letting Slater go if he hands over his weapon to Antonia, and although Slater refuses at first, he eventually passes it to his colleague. Sam then speaks to Antonia alone and tells her they may both live through this, but he doesn’t intend for Slater to survive. Sam says he’s already killed someone for her, and now he wants her to return the favour and kill someone for him. He promises to forgive her for everything if she agrees to the deal.

The Doctor and Menzies are struggling to find the bomb in the pitch dark of the basement, so the Doctor calls for Sam to turn the lights on. When they find it the Doctor searches through his pockets for an etheric beam locator. He warns Menzies that he doesn’t stand much chance of stopping the countdown, but it should be safe to open the briefcase. He examines the device and deduces that it’s been programmed with the precise shape and size of the entire building. However, in order to operate effectively, the bomb has also been programmed to account for changes in the building’s state, so he begins reflecting the internal tracking sensors back on themselves. The bomb will think the building has gone and that there’s nothing left for it to blow up. The timer reaches 20 seconds and the Doctor counts the seconds down until it reaches zero. There’s a small explosion and the Doctor congratulates himself. There’s no weapon he likes better than one that destroys itself.

The Doctor and Menzies return to Maxine and thank her for her help. Just as they’re wondering what happened to Slater, the lift arrives on their floor and the doors open. Charley emerges in a state of shock and tells the Doctor that she’s just seen something terrible. Antonia places the gun down on the floor and follows her out. She confesses to her crime, but points out that Slater deserved everything he got. Charley disagrees strongly and says that isn’t the way to settle things. Menzies places Antonia under arrest and as she’s led away, Antonia apologises to Sam. Menzies asks the Doctor if he’ll come back to the police station with her as they still need to round up Slater’s friends and she‘ll need his help to fill in all the blanks. Menzies knows her experiences have changed her life, but she still has a job to get on with. Just then, Menzies’ phone rings and DCI Turnbull breaks the news to her that he’s just discovered Slater knows the murder victim, Gregory Bailey. He seems to think the mysterious Doctor is the key to everything as it all leads back to him. He asks if the Doctor is still with her, but Menzies says he isn’t. She hangs up and warns the Doctor that her boss is looking to pin the whole thing on him so it’ll be simpler for everyone if he just disappears and then she can give Turnbull her version of events. But first she needs to sit down with a pad and a pen and work out what she’s going to say! Maxine is still concerned that the building really is haunted and she hopes it’s a friendly ghost, but Charley reminds her that he did commit a murder. Maxine wonders what will happen if the building ever gets knocked down, but the Doctor isn’t sure and says it could destroy him. Maxine decides to make sure that’ll never happen.