Inside the TARDIS control room, Charley finds the Doctor dressed from head to toe like an Eskimo. He tells her it’s genuine Inuit garb that he borrowed from the explorers Robert Peary and Matthew Henson. He’s convinced they’ve arrived inside an ice cavern and he’s determined to begin his climb from the icy crevasse. Seconds later, he returns and tells her they’ve landed in a jungle. Confused, he checks the instruments, but they confirm the temperature outside should be -22º F, not the sweltering heat he’s just experienced. He decides to run a systems diagnostic while Charley looks around outside. The Doctor discovers everything is working at 98% effectiveness, but then the figure starts falling rapidly. He hears a noise from outside that he recognises and when Charley cries out, he rushes out to join her.

Charley has been sprayed on the leg by one of the nearby plants. The Doctor urges her to return to the ship immediately so he can stabilise the fungoid infection, but then the plants spray the doors to the TARDIS, preventing them from entering. The sky suddenly becomes dark and within seconds it’s night time. To Charley’s surprise, the Doctor calls out to the invisible people who live here, asking for their help in curing the infection. He tells her they’re on Spiridon, a world in the ninth system of a far-off galaxy with an icy core, which is tropical by day and sub-zero by night and carpeted with strange and deadly flora.

The nearby bushes start to move apart, but the people who emerge are not the invisible natives they expected, but a group of heavily armed Thals. Their commander, a woman named Tamarus, orders Valion, Jesic and Septal, to search the new arrivals. The Doctor asks them to help Charley, but Valion refuses to waste any of their medical supplies on non-combatants, although he does offer to amputate her leg. The Doctor explains that the Thals are undoubtedly here to fight their blood enemies, with whom they share their home planet Skaro. The Doctor notices Charley’s reaction to the name Skaro, but before he can question her, they’re distracted by the sound of a space orbiter flying overhead.

The Thals prepare to fire a flare into the sky to attract the attention of the pilot, but Jesic discovers their entire supply of flares are wet and won’t work. The orbiter passes by and the group is resigned to the fact that they’ll have to spend another three nights in the jungle before the ship passes this way again. Valion is convinced they can’t survive that long, but it also becomes clear that they can’t even predict exactly when the orbiter is due to pass. The group is lost and the ship is on its way to where the platoon should be. Tamarus reluctantly admits that their positioning equipment hasn’t been working recently, but with her platoon depleted and demoralised, she’d judged it best not to burden them with that information. She says she’s confident she can still calculate their location without exposing them to their enemy, but they’ve already had one encounter with a passing patrol, which resulted in the death of their colleague Nyaiad.

The group starts to argue, with Valion blaming Tamarus for Nyaiad’s death. He raises his weapon towards her, so she orders Jesic and Septal to kill him if he fires. Charley believes Valion is suffering from some form of shell shock and when the Doctor intervenes, Valion accuses them of being spies. Jesic notices that although Charley was infected by one of the plants, her leg hasn’t developed any blisters. They suspect she may be an enemy replicant and even the Doctor agrees that the lack of symptoms is odd. Charley insists that she’s from Earth, but the Thals regard the name as something from one of their fairy stories, like the words ‘mutos’ and ‘Doctor of TARDIS’.

The Doctor and Charley realise they won’t be believed even if they tell the truth. Instead, the Doctor claims to be an amateur botanist, but Charley inadvertently complicates matters when she reveals that he’s visited this planet before. Jesic wonders how they came here, but when Charley tells them about their blue box, Jesic is suddenly flooded with strange memories in which the Daleks chant the word TARDIS. Tamarus urges Jesic to snap out of her trance. The Doctor has no choice now but to reveal that he and Charley travel through time and space, but when he tries to show them where the TARDIS landed, they discover it’s no longer there. Septal believes the Spiridons must have carried it off, but the Doctor knows they couldn’t have done that without being heard. Instead, he thinks his ship has been absorbed by the jungle.

Valion insists again that the Doctor and Charley might be replicants, designed by the Daleks to infiltrate and kill. Charley accuses them of seeing Daleks in their own shadows and the Doctor notices that she seems surprisingly familiar with the word. It suddenly occurs to him how effective the Daleks’ strategy of replacing people with copies can be. Jesic tells Tamarus she has the strange notion that she remembers Charley from somewhere - and she even recalls that her name is Charlotte Pollard…

Elsewhere on the planet, a young woman contacts Comrade Director Murgat and reports that there’s been a regression in the jungle. He assures her it should auto-correct and asks her to log the incident so they can evaluate it later. When she also reports that she’s picking up signs of foreign bodies, Murgat thanks Nyaiad and asks her to check whether the sensors have failed.

Jesic insists that Charlotte Pollard doesn’t belong here, then she collapses. Tamara and the others try to revive their friend, but they refuse the Doctor’s offer of assistance. Charley suggests they use the opportunity to escape while the Thals are distracted, but when he advises against it, she runs off into the jungle alone. Valion chases after her and the Doctor runs off in the same direction, followed by Tamara.

Charley stops when she realises the Doctor isn’t with her. She hears movement in the nearby bushes and calls out to the invisible people, explaining that she was “splotted” by one of the plants and asking for their help. Suddenly the bush itself seems to come to life and Charley finds herself being eaten alive…

The Doctor finds himself by some ruined buildings. Valion arrives shortly afterwards, but the Doctor refuses to be cowed by his weapon and digs deep into his own pocket to retrieve an ice axe. He uses it to scrape the moss away from the stonework so he can read the inscription underneath, but to his surprise, he discovers the hieroglyphs come to an end as soon as they reach where the moss was. It’s clear there was never anything carved there in the first place, despite the fact that the exposed writing only makes half a sentence. They hear gunfire from nearby and race off towards it…

Nyaiad continues to fire until the plant creature that was attacking Charley is disabled. Charley is grateful to be rescued, but concerned that the woman, who appears to be wearing some sort of gas mask, is still pointing her weapon at her. Charley introduces herself and Nyaiad is surprised that she claims to have seen others like her. Then, to her horror, Valion comes racing out of the jungle and literally tackles her to the ground. They struggle for a while, but then the woman addresses him by name and he releases her. He rips off her gas mask and is shocked when he sees who she is. She pleads with him to return her mask, but then the Doctor arrives and drags Valion away. Nyaiad introduces herself and the Doctor remembers hearing that she was supposed to be dead. Valion believes she must be a Dalek replicant and insists that he saw her shot down by a Dalek during an ambush. He becomes confused and waves his weapon at everyone. Nyaiad says his trauma is so deep, she’s not sure it can be overcome. She orders him to lower his gun and return her mask and eventually he agrees, but Nyaiad discovers the mask is broken.

Suddenly Tamarus arrives and when she sees Nyaiad, she too accuses her of being a replicant. The Doctor pleads with her not to take any hasty action, but Tamarus doesn’t listen and shoots Nyaiad dead. Valion rebukes his commander for killing their friend, but Tamarus reminds Valion that he once was a veteran of the Mechanoid Wars and says he’s now become a disgrace and embarrassment to his own platoon. Valion is furious at the accusation and shoots down Tamarus. He insists that she wasn’t his real commander and that she was a traitor. He struggles to find the right word to justify his actions and in his mind he imagines he can see a group of Daleks, also searching for the word…

Valion collapses and when Charley asks the Doctor what word he was trying to say, he suspects that she may already know. Septal arrives and is surprised to see Valion unconscious, Tamarus dead and Nyaiad apparently back from the grave. Nothing seems to make any sense, but the Doctor thinks he knows the answer. He reveals that they’re not on Spiridon after all. It was the moss-covered hieroglyphics that finally proved it - the writing wasn’t complete because the words hadn’t been perceived by the Thals to exist. Septal thinks the Doctor is trying to confuse him with psychological warfare and tries to resist. Suddenly Charley notices the infection on her leg is starting to spread, just as everyone said it would, but the Doctor assures her it isn’t and that it only appears to be spreading. Until now, the dominant impression of her was that she was a Dalek replicant, but now Valion is unconscious, the other Thals’ mental engrams are re-shaping everything. With only Septal present, the Doctor believes they can take charge of the environment.

Before the Doctor can respond, the entire chamber is lit up, dazzling his and Charley‘s eyes. To their surprise, Commander Tamarus enters with Jesic, Septal and Valion, all of whom are wearing gas masks. Tamarus orders the others to secure the experimental units. The Thal-Daleks are confused and refer to their counterparts as comrades, but Septal switches them off. Tamarus demands to know how the Doctor and Charley broke into this facility and when they don‘t answer she orders Jesic to strike Charley. The Thals discover Nyaiad is dead and when Tamarus asks the Doctor if he was responsible, Charley points out that it was actually Tamarus herself who did it.

The Commander leads the group out of the chamber and tells her colleagues to be careful with Nyaiad’s body as they might still be able to retrieve some of her personality. Charley isn’t sure that what they’re experiencing is real, but the Doctor points out that she’s bruising from when Jesic struck her. She argues that if the jungle was just a fantasy generated by mental engrams, then so was the anti-hallucinogenic flower that woke them up and the Doctor admits it was just a ruse to help her shatter the illusion. He tells her they’re not on Spiridon, but in the hollows of an ice planet, just as the TARDIS said they were.

The group passes through an airlock and once the air is cleared, the Thals remove their gas masks. A voice calls out to Tamarus to bring the strangers through and the Doctor and Charley are amazed to discover the Thals are being led by a man who is half-Thal and half-plant. The Director, who introduces himself as Murgat, is living inside a translucent casing, like a personal greenhouse, with water constantly being pumped through to keep his foliage moist. Murgat sends the other Thals to resume the experiment, then he asks the Doctor and Charley to accompany him.

In the main laboratory, Jesic tells Tamarus she thinks Murgat is improvising, but the Commander says they‘ve already gone far beyond the parameters of the original experiment and they‘re breaking new ground. They reset the test environment to day one and revive all five test subjects, including the Dalek version of Nyaiad. They start recording as the Dalek version of Tamarus orders her platoon to fall in…

In the fictional Spiridon jungle, Tamarus warns her colleagues their numbers aren’t sufficient for them to engage with the enemy. This mission is for reconnaissance purposes only, but Valion rebukes the others for not taking things seriously enough. Tamarus orders him to take Nyaiad and go ahead, while she, Jesic and Septal follow behind…

Director Murgat takes the Doctor and Charley to another chamber which stretches out for over 100 acres and is filled with more of the purple plants. It’s clearly a hothouse, with the temperature and humidity strictly regulated, and Murgat tells them the plants are native to an obscure unnamed world four galaxies away. Charley yells out as she brushes against a small primate, like an oversized lemur, that was hiding in the undergrowth. Murgat assures her it won’t hurt them during the daylight cycle and the Doctor recognises it as a Jekyll, from the planet known only as YT45. This means the plants they’re harvesting here must be kyropites!

In the jungle, Jesic breaks radio silence and tells Tamarus she’s found something - a blue box that obviously doesn‘t belong here. She’s not sure why, but somehow she knows it’s important. In the main laboratory, the Thal Jesic detects a neural spike on her equipment and thinks it might be caused by a query regression. They watch as the Dalek Jesic concentrates on the image of the blue box and starts to go into a trance. Then suddenly she’s once again flooded with strange memories in which the Daleks chant the word TARDIS.

The Doctor is keen to move clear of the kyropites, but Murgat tells him they’ve all been sealed in to maintain the humidity. He assures them they’re perfectly safe and, to prove he’s telling the truth, he removes his translucent head-piece and takes in a number of deep breaths. The Doctor realises the man is part-kyropite himself, but that doesn’t mean he and Charley aren’t in danger. He tells Charley that the pollen from the plants lulls their victims into a dream state, during which they live out a fantasy existence while slowly being turned into compost. Murgat says the Jekylls have a symbiotic relationship with the kyropites, guarding them by night in exchange for immunity to the poison, but he‘s able to extract their immunity for himself. The Doctor and Charley seem to be alright too, but for all they know they may still be trapped inside another prison of their own fantasies. Murgat points out that the pollen is being sucked out by huge extractor fans and then pumped out into the ice cavern, which is how he’s able to convince the Daleks that they’re Thals. The Doctor wonders how he and Charley were able to share the illusion of being on Spiridon together, but Murgat says he has more to show them…

Night has fallen fast in the fictional jungle of Spiridon. Valion surveys the area using infra-red lenses, but all he can see are ruins. He warns his comrade Nyaiad to stay alert and not to be distracted by the stars, but the way he talks to her about the planet makes her suspicious as she knows he’s never been here before. Tamarus contacts them and says Jesic has found a blue box not far from their position. Valion has another flashback - this time to when they first met the Doctor and Charley - and in the laboratory, the real Jesic registers another neural spike on the instruments. She suggests they contact Murgat…

Murgat tells the Doctor and Charley this was once an experimental Dalek facility, but with the Doctor’s help, the Thals were able to drive them out of this corner of the galaxy. The Doctor knows he hasn’t been near Antares in six millennia, but then he realises it may not have happened yet. Charley points out that it could be disastrous to the Web of Time if he learns about events in his own future, which the Doctor finds curious as he’s sure he’s never discussed such matters with her before. He’s about to question her about it when Murgat reveals yet another secret - the bodies of Tamarus, Valion, Jesic, Septal and Nyaiad - all unconscious and with their faces completely covered by one huge kyropite. This explains how the whole platoon is able to share the same dream, but Murgat tells them he’s also having the dream relayed electronically into the brains of the Daleks in the ice cavern.

The Doctor is horrified and prepares to unhook the electrodes attached to the Thals’ brains so they can be restored to the real world, but Murgat warns him he’ll also wake up five very angry Daleks. The Doctor refuses to condone such a sadistic experiment, but Murgat says it’s actually the Daleks own experiment. It’s the Thal platoon who were originally kidnapped by the Daleks on Spiridon, and by the time the Thals captured this facility, the experiment was already underway. Murgat did try to release his compatriots, but was himself infected by the kyropite. Fortunately he was able to improvise a solution so the work could continue. The Daleks have previously experimented with mind control, but only succeeded in creating robotised drones - but on this planet, they were planning to use the kyropites to transmit their own thought processes into other people’s brains so they could enslave an entire species. The aim was to pollinate entire worlds with their ideas even before their saucers landed. Murgat says there’s nothing he can do for the Thals here as their minds are now trapped in their own trauma, but he can still help them reap their revenge on the Daleks.

Charley likens it to a Fifth Column, a crack squad of Thal soldiers whose personalities have been transmitted into the Daleks brains, who would then be sent out on a mission to sabotage the Dalek war machine from the inside. The Doctor is certain it won’t work and says the personality grafts have been too crudely spliced, leaving the Daleks with only the intonations and inflexions of their originals. Charley points out that if the unconscious bodies are the original platoon from Spiridon, then who are the people they met earlier? Murgat reminds them that this was a Dalek facility and says the versions of Tamarus and the others they’ve encountered are replicant clones.

Elsewhere in the base, Valion and Septal oversee the resurrection of another Nyaiad clone. The young woman splutters and coughs as she emerges from a fluid tank. She greets her comrades and provides her name, rank and generational number. She’s declared fit for duty, but when she tells them she remembers being shot, Septal explains that she was “melted down” and then reconstituted. However, she also has a recollection at the back of her mind about meeting the Doctor and then suddenly she’s flooded with strange memories in which the Daleks chant the Doctor‘s name and the word TARDIS.

The Doctor tells Murgat he wants nothing to do with what’s going on here, but as he prepares to leave, Charley notices the unconscious Thals are whispering to each other. Murgat assures them it’s just muscle spasms, but Charley insists the Thals are arguing amongst themselves. The Doctor confirms that she’s right as he can make out them repeating the words Doctor and TARDIS. The revelation shocks the Doctor as he finally realises what Murgat wants with him.

In the laboratory, Jesic reports that four of the Thal-Daleks are now showing signs of multiple regression. Urgently Tamarus contacts Murgat and asks what they should do, but he simply suggests they increase the flow of kyropite pollen and tells her to remain calm.

The Doctor realises Murgat deliberately left the TARDIS in the jungle with the Thal-Daleks in order to test them. He believes the experiment has failed as the illusion has now been shattered - one sniff of their eternal enemy was all it took to make them revert to their heartless Dalek state. Charley isn’t so sure as she listens carefully to what the unconscious Thals are saying…

In the jungle, the Thal group have convinced themselves that the TARDIS is just a fairy story, like mutos and humans from Earth. They believe the jungle is playing tricks on their minds. Jesic argues that the blue box is real, but she soon discovers she’s the only one who can see it now.

Murgat is satisfied. A small increase in pollen count throughout the ice cavern is all it took to overcome the most deep-seated of the Daleks’ hatreds. He believes the process works, but the Doctor is sure it won’t last. Murgat says the Thal-Daleks need a leader, a tactician with experience of fighting the Daleks, and he intends that the Doctor will travel with them into the heart of the Dalek empire to destroy it from within. He summons the clones of Valion, Septal and Naiad to the hydroponics section and tells the Doctor he must join the Thals in the kyropite dream. If he resists, Murgat will order the clones to inject the kyropite toxin directly into his brain. Charley points out that Murgat and the Thals are constantly referring to each other as comrades and brothers, and she says she’s met their sort before. The Doctor agrees as the language they’re using isn’t normal for Thals, but Murgat says they’ve learned it from books like “Das Capital”, whose truth has transcended the bounds of time and space. He says the Daleks stand on the right, for exploitation and conquest, so the only way to defeat them is to stand with the heroes of the left. The Doctor wonders whether Thal High Command knows what they’re doing here and Murgat admits that he’s been somewhat neglectful in keeping them informed.

The Doctor gets impatient while waiting for Charley’s explanation, so eventually she agrees to come straight to the point…but before she can reveal anything, she notices the extractor fans have stopped working. The kyropite pollen starts building up, so the Doctor urges Charley to cover her nose quickly. Murgat calls for assistance in re-sealing his casing, but when Charley wavers over whether she should help him or not, the Doctor says they don’t have time to play games and helps Murgat back into his personal greenhouse.

The three of them hurry for the nearest exit, then they seal the entrance to the hydroponics section. Charley inadvertently makes a reference to the Doctor having met Sigmund Freud and when he denies it, she realises it hasn’t happened to him yet. Valion arrives and Murgat tells him to restrain Charley while the others repair the filtration system. Murgat has realised the Doctor doesn’t know Charley as well as he thought he did, but as they argue, Charley manages to escape from Valion’s grasp. Unfortunately she runs straight into the new clone of Nyaiad who narrowly misses shooting her. To Murgat’s surprise, Nyaiad reveals that she was the one who shut the filtration system down. She struggles to understand why she keeps thinking of the blue box and the Doctor points out that everything seems to revolve around her. Charley says the real Nyaiad died on Spiridon in a Dalek ambush, so who is this one? The clone of Valion has a flashback to the moment when he saw Nyaiad killed and the memory is traumatic for him. Suddenly the truth becomes clear to him - Nyaiad was a Dalek replicant from the very start. This changes everything and the Doctor tells Murgat to shut down the experiment immediately. Valion asks Nyaiad if this is true, but she responds by shooting him dead on the spot. Murgat uses the opportunity to escape, but Nyaiad manages to take the Doctor and Charley prisoner.

In the fictional jungle, Valion shivers as if he felt something walk over his grave. He dismisses Nyaiad’s concerns and tells Tamarus they have to go on with their mission. Their commander is reluctant to move on as she and Nyaiad realise something is very wrong here. Nyaiad points out that this place is cold, empty and hollow, and she’s surprised the others can’t feel it…

As Nyaiad escorts the Doctor and Charley through the facility, the Doctor tries to convince her she may not be who she thinks she is. Nyaiad stops when she hears voices inside her head and she responds by saying she’s never heard of Folkestone. The Doctor suspects she’s having an attack of consciousness and it’s almost like she’s finishing off someone else’s conversation. He tells her she’s trapped in a loop, not knowing where the real person ends and the other version begins, but she has another flashback of the moment she was killed by the Daleks and finds herself struggling to remember a particular word. The Doctor believes the Dalek weapon kick-started her secret identity as a Dalek replicant and that she was programmed to infiltrate the Thal platoon. Nyaiad says she has standing instructions to capture the Doctor and take him to her masters, but the Doctor asks her if she knows what happened to the real Nyaiad. Perhaps she was captured alone in the jungle and interrogated?

Nyaiad has another flashback, to a time when she was being questioned by the Daleks in their underground base on Spiridon. Then, when she refused to co-operate, she was tortured…

The Doctor wonders if the recollection means she was a Dalek replicant first or is she the last repository of the consciousness of a the original Thal? Perhaps she was tortured and murdered by the Daleks and her memories pasted into a crude cloned body? As she leads them back inside the cavern representing Spiridon, she puts on a gas mask in case there’s any kyropite pollen remaining inside. The Doctor tells Nyaiad it’s time she decided who she really is and to whom her loyalties lie, but she tells him she’s loyal to her brother comrades. The Thal-Daleks arrive and demand to know who the newcomers are and what they’re doing in this place. Nyaiad pleads with them to lower their weapons. While all this is going on, Charley tries to persuade the Doctor to stop being the compassionate hero and join her in escaping, but he’s disappointed by her attitude. Charley points out that the Daleks might still revert to type and kill them, but even if they don’t, they might still regain enough cognisance to wreck Murgat’s experiment.

Outside the hydroponics section, Murgat tells Jesic and Septal to lock the bulkhead so no one can escape. Then he tells Tamarus to put on her gas mask and carry out the orders he’s given her. She agrees, albeit reluctantly, but he rejects her request to switch the lights on to protect her from the Jekylls as the environment is in its night cycle and he can’t change it.

The Doctor and Charley watch as the Thal-Daleks insist that they are the real versions of Tamarus and Valion. Nyaiad tells them they’ve been deceived and says she’s come to free them from their prison. The Doctor steps forward and tells the Daleks not to listen to her, but then Nyaiad reveals that the Doctor is their mortal enemy…

In the hydroponics section, Tamarus searches for the Thal dreamers. The Jekylls don’t appear to have seen her yet, but she proceeds with caution. She eventually finds the unconscious platoon and Murgat orders her to remove the neural relays from Nyaiad’s brain, even though he knows it will kill her as well as the replicant who’s slave to her consciousness. She removes the jacks going into Nyaiad’s brain and then apologises to her friend. Suddenly Nyaiad reaches out and grabs Tamarus’s wrist, then Tamarus spots Jekylls all around her. She’s convinced they know what she came here to do, but Murgat says that’s impossible. Murgat listens helplessly as the primates launch at Tamarus and begin tearing her apart. He calls to her, but after a few screams, the communicator goes dead. Murgat realises the Jekylls not only protect the kyropites, but also anyone who’s been infected by the plants.

In the jungle, the Doctor hopes the Thal-Daleks are more Thal than Dalek and he tells them he’s been a friend to their people in the past. The creatures confront him and say there’s no such thing as the Doctor and the stories about him are nothing more than fairy tales told for children. They begin to laugh in their sinister mechanical voices. Nyaiad assures them he is the real Doctor, but the Thal-Daleks order her to step closer and remove her gas mask. She asks them if they recognise her, but all it does is confuse them further as they know that Nyaiad is already dead. They convince themselves that Nyaiad is a Dalek replicant and they turn to destroy her. Nyaiad tells them to look at their own reflections in the ice, but when they do they see only their replicant Thal bodies staring back at them. The Doctor tells Nyaiad their conditioning is too strong and she’ll never break it. The Thal-Daleks start arguing over what to do, but when Tamarus moves to kill Nyaiad, the other Thal-Daleks conclude that she must herself be a Dalek replicant because destroying people is not the Thal way. In unison, the Thal-Daleks turn on Tamarus and blast her down with their Dalek guns. Nyaiad starts to cry and the Thal-Daleks comfort her by assuring her they’ve only eliminated a Dalek replicant. As he watches her, the Doctor is finally convinced that she genuinely is the last remaining remnant of the original Thal lieutenant.

The Thal-Daleks welcome Nyaiad back to their brotherhood and convince her she’d been captured and tortured by the Daleks, but that she escaped to warn them there was a replicant in their midst. Charley suggests to the Doctor that they might leave now as the Thal-Daleks seem to be stable and the last thing they want to do is reawaken their memories of him again. This reminds the Doctor that the replicants had memories of Charley too from some previous encounter in Folkestone. He suggests he make contact with the people on Skaro and get Murgat’s unethical experiment closed down for good. Although he’s reluctant to leave these poor people to their delusion, he sadly agrees with Charley that there’s nothing they can do here.

The Thal-Daleks help their recovered comrade Nyaiad back to their base in the jungle. Now that she’s established a Dalek presence on Spiridon, they believe their mission has been accomplished and that they’ll soon be going home. Septal goes ahead and calls out that he’s found the rendezvous site. As he sets off some flares, Nyaiad’s mood changes. Speaking in a strange monotone voice, she says her mission has been accomplished and that the experimental cycle has now concluded. The others gather around her, concerned, and they begin to suspect that she’s responding to something beyond her hearing.

In the laboratory, Jesic calls over Murgat, but the only thing he’s interested in is whether the experimental subjects have killed the Doctor. Jesic reports that the Daleks have stabilised into their Thal identities, even though the pollen levels in the cavern are negligible. Murgat declares the experiment a success and his colleagues congratulate him, but Septal is confused as the Daleks are still experiencing something strange…

The Doctor and Charley make their way back to the TARDIS and bid farewell to the Thals and the Daleks. The Doctor reminds Charley that both races are descended from the same species and he wonders if it would be bad for the Thals to regain some of the Daleks’ ingenuity and instinct for survival and for the Daleks to regain some of the Thals’ comradeship. As they step inside the ship, they hear a loud bang. Could it be the beginnings of a tropical storm, or an ice cannon, or a glacial movement? The Doctor recognises the sensation of static electricity in the air and he races back out into the cavern and points to a mile-wide crack in the ceiling.

The Thal-Daleks prepare for the return of their orbiter. They watch as the survey pod descends, then Nyaiad announces that her real orders are to contain the facility and seek and apprehend all the experimental subjects. Valion realises, to his horror, that the approaching ship isn’t their orbiter and he warns everyone to take cover. He now knows they’re not in a jungle at all and that Nyaiad is receiving instructions from their enemies.

In the laboratory, Jesic and Septal turn to their Director for reassurance, but Murgat is equally confused. He sends an emergency message to Thal High Command, warning them that the YT45 installation has been infiltrated by the enemy. However, when the response arrives, it’s not from the Thals, but from the Daleks. They tell him his facility was always infiltrated from the very beginning.

As the Dalek ship descends down onto the surface, gas is released into the air. The Thal-Daleks start to panic and split up, racing into the trees to get whatever cover they can - but suddenly they discover there are no trees anywhere in sight. The ship touches down and a Dalek containment squad prepares to disembark.

The Doctor orders Commander Tamarus to get her platoon away from here using a doorway that leads into an experimental facility, but the information is too much for her to take in. Instead, he turns to Valion, but the officer is now starting to realise he and his entire platoon are Daleks. The Doctor tries to convince them they’re better than Daleks, but they plead with him to destroy them. Eventually each of the Daleks comes to a halt, leaving just the Doctor and the replicant of Nyaiad. She takes him prisoner just as the Dalek containment squad approaches, headed by the Black Dalek himself.

The Doctor can’t believe what he’s hearing, but Charley says she’ll only speak to the Black Dalek. Both of them are taken away for interrogation and the Doctor is so shocked by what he‘s learned, he doesn’t resist. Moments later, Murgat is brought before the Black Dalek. He insists they won’t get anything from him by using force, but the Daleks tell him he’ll be destroyed if he resists. The Thal replicants are horrified to discover they’ve been working for the Daleks all along and Murgat is forced to admit that he’s not sure of anything any more. The Daleks have no further use for the Thals and prepare to kill them, but Murgat insists they’re more than just replicants and they could still be useful as experimental subjects. He reveals that he’s discovered the trauma of death and injury in the replicants can be experienced by the Thal-Daleks, and he suspects the opposite may be true too. He wishes to injure the Thal-Daleks in order to cause the replicants pain as the logical next step of the experiment. The replicants protest, but Murgat tells them it’s no use asking the Thal-Daleks for help as their minds are now broken. Not far away, the Thal-Daleks can overhear the conversation and they watch as the Daleks escort Murgat and the replicants aboard their spaceship.

Inside the spaceship, the Thal replicants believe they’re being taken to the central Dalek laboratory back on Skaro, but the Doctor has already realised there are other areas of the experimental facility they’ve not seen yet. Murgat is surprised to discover there was more than one level to the facility, but the Black Dalek confirms this and tells them they’re leaving Zeg-1 and heading for Yarvell-6. This was where an experiment is being conducted to determine the resistance to external stress of Dalekenium alloy. They watch on a scanner as the Daleks melt some of their own kind inside their casings. The test subjects had been damaged in conflict, but at least here they can continue to serve the Dalek cause. Charley is horrified, but the Doctor says the Daleks would call it progress, and who knows how many other levels there are where similar experiments are taking place? The Black Dalek turns to challenge Charley - she claims to be a Dalek replicant, yet she expresses sentiment and feelings. Charley blames the Doctor and says his emotions must have rubbed off on her after her travels with him. Murgat agrees and says his experiments have shown the replicants are susceptible to this.

The Doctor realises the point of the experiment was to create Daleks who think like Thal soldiers in order to capture the spirit of comrades in arms - anything that might give the Daleks an advantage, however small, on the field of battle. Murgat says the Thal subjects would never have co-operated if they knew their efforts were helping the enemy. The Black Dalek orders the Thal-Daleks to be taken to the central laboratory, but the creatures refuse to co-operate and, on the instructions of the Dalek who believes herself to be Tamarus, they open fire on the Daleks sent to accompany them.

In an interrogation room, Charley continues insisting she’s a replicant, but the Daleks have no record of anyone being sent to capture the Doctor. She’s ordered to place her hands on a truth detecting device, but she tells them it won’t work on her because she’s more advanced than the crude clones of this century. She claims to be from the year 500,002 and says she tricked the Doctor into letting her accompany him and has been spying on him ever since. Her mission was to gain a unique insight into his mind so the Dalek Supreme could identify and cure the psychological weaknesses in the Dalek race the Doctor has exploited over the years. The interrogating Dalek accepts that her story is plausible (which actually surprises Charley for a second), but the Black Dalek, who’s been monitoring the conversation, isn’t so sure and challenges the Doctor with Charley’s story. The Doctor admits that he’s been suspicious about Charley and that he knew something wasn‘t right about her. The Black Dalek seeks advice from Murgat, who tells him a replicant of such sophistication is beyond current Dalek technology, but it may well be possible in the future.

The Black Dalek believes there’s another explanation and tells them to observe the replicant Jesic in an adjoining interrogation room as she remembers a previous encounter with Charley in Folkestone on Earth. The Black Dalek is about to reveal details of the earlier Dalek invasion, but the Doctor doesn’t want to know as these events haven’t happened to him yet - but the Black Dalek points out that they do seem to have happened to Charley. The Daleks decide to interrogate the Thal-Dalek that was imprinted with Jesic’s memories, but they discover the whole group are missing. The Doctor whispers to Murgat and tells him the Daleks won’t tolerate him for long, so Murgat offers to investigate what’s happened with the Thal-Daleks. The Black Dalek tells the Doctor they’ll learn the truth eventually, and when they do they will destroy him!

Charley is delighted when the Doctor joins her in the interrogation cell as it feels like it’s been days since they last saw each other and she was worried they might have done something terrible. Charley suspects the Daleks know she’s lying about being a replicant, but she says it was worth a try. He tells her the Black Dalek’s theory is that she was travelling with one of his future incarnations and that she’s broken the First Law of Time by hitching a lift with him. At first Charley disputes this, but then she admits that she met the Doctor on the airship R101 and that he saved her when she really should have died. She begins to wonder if this is the Web of Time’s way of getting revenge on them both. She tells him about the wonderful adventures they had and how she never wanted it to end, but then in the year 500,002 she saw him die. The Doctor suggests the Time Lords could have addressed the problem, but she begins to get upset and says she wanted him back more than anything. She realises the Doctor is probably angry with her and says she wouldn’t be surprised if he wanted to kill her, but her words seem to trigger a strange reaction in him and he begins repeating the words “kill you”…

Accompanied by the Thal-Daleks, Murgat returns to the Black Dalek and learns that the personality graft on the replica Doctor is failing. Murgat isn’t surprised and says the Dalek replicant technique is too crude. He believes this is why his own technique of using the kyropites to stabilise the personality is so much more effective. Nevertheless, the experiment has succeeded in forcing Charley to reveal the truth and the Black Dalek believes they can destroy the Doctor‘s future by replacing her with a replicant under their control. Murgat is ordered to begin work on creating a replacement companion, then the Black Dalek calls for an execution party to kill the real Charley. The Thal-Daleks refuse to obey their orders and reveal that they’ve come to rescue their comrades. The Black Dalek calls for assistance, but the other Daleks also voice their support for the Thal-Daleks and agree to join them. Murgat explains that he’s released kyropite pollen into the air-conditioning system and the Daleks now believe the Black Dalek to be decadent. On the orders of the Thal-Daleks, the Daleks turn on Black Dalek and destroy him. The Thal-Daleks call on their brother Daleks to rise up and join them in mutiny against their rulers.

In the cell, Charley has realised the Doctor is a Dalek replicant and demands to know what they’ve done with the real Doctor…

The TARDIS materialises in the Spiridon jungle and the Doctor emerges with his companion Jesic. At first the Doctor thinks they might be in the Congo or in darkest Peru and he warns Jesic to close the door before any of the local fauna gets in. Without warning, the Doctor’s coat is “splotted” by one of the plants, then they notice how dark it’s getting. They both realise they’re on Spiridon, a world in the ninth system of a far-off galaxy, which is tropical by day and sub-zero by night and is carpeted with strange and deadly flora. This is the planet where they first met all those years ago when the Doctor was travelling with someone else, whose name he can‘t remember. Suddenly several Dalek emerge from the jungle and the Doctor notices they all have the Star of Lenin embossed on their domes. He assumes they’re going to be taken prisoner, but then one of them speak with the voice of Tamarus and addresses them as their brothers and comrades. Together the Daleks chant that they want the travellers to join them.

Just then, Jesic spots someone watching them from behind the sponge plants. Charley emerges, wearing a gas mask. The Daleks tell her humans are welcome to join them too, but the Doctor wants to know who she is and what she’s doing here. She removes her mask, but the Doctor still doesn’t recognise her until Jesic prompts him. Jesic is convinced Charley’s a replicant traitor and urges the Doctor to remember the time when she betrayed them. The Doctor slowly starts to realise that not everything is as it should be, but he turns to the Daleks and urges his “brothers” to kill Charley, but the Daleks refuse to kill someone without a reason. The Doctor asks them to remember the word and says if they were proper Daleks they’d know what word he means, but the Daleks ignore him and say they believe in unity, solidarity and common ownership of the means of production. The Doctor tells them their brotherhood is the fantasy of a man named Murgat, who read one book and thought he’d found the answer to universal peace.

Jesic is still suspicious about Charley and the Doctor asks her how long he and Jesic have been part of this fantasy. Charley says she doesn’t know and that she’s only just found out herself. Knowing that Murgat is bound to be listening, the Doctor calls out to him and warns him that the Daleks will never change their spots. He tells the Daleks that he’s defeated them time and time again on a thousand worlds and that he’s destroyed whole battle fleets, but the Daleks still refuse to believe he‘s their enemy until he mentions their encounter in Folkestone. They suddenly realise he’s their enemy and he tells them he’s at their mercy, goading them into taking their best shot. True to their nature, the Daleks form into an execution squad. Charley pleads with the Doctor to take care, but he says he’s sure Murgat will step in to stop them as he wouldn’t want the brotherhood of the Daleks to start taking lives without good reason.

The Doctor shouts to the Daleks to fire at him - but just as they chant the word ‘exterminate’, their gun sticks drop and the creatures become inert. The jungle reconstruction disappears and the Doctor, Charley and Jesic find themselves back in the huge ice cavern. Charley explains that Murgat was conducting one final test to see what the Daleks would do when they met the Doctor again. Murgat arrives in the cavern and says the Daleks rescued him from the Black Dalek, then they decided to live neither as Thals or Daleks but as children of the revolution. The Doctor is sceptical and prefers to believe that their minds simply broke under the strain and all that was left was Murgat’s political cant. Murgat calls for Septal to “open the sky” and the huge roof opens up above them.

The Doctor warns him the Daleks will never change, but then they can hear the voices of the Daleks singing the anthem of the Bolshevik Party. The Doctor is momentarily stunned, but Charley says she’s also heard their version of the Internationale. The Doctor asks how tame the brotherhood would be if the atmosphere hadn’t been flooded with kyropite pollen, but Murgat says he plans to wean them off the pollen in stages. The Doctor knows he can’t lie to them forever and sooner or later they’ll revert to type as they nearly did just now. Murgat accuses the Doctor of provoking them into remembering things they’d rather forget, but the Doctor is worried what they’ll do when they find out they’ve been deceived. Murgat says that one day their philosophy might be sufficiently advanced to let them know where they came from and then he plans to let them out into the Universe to spread comradeship and brotherhood.

Charley thinks this sounds reasonable, but the Doctor is convinced it’s a useless dream. He says he can’t allow a new generation of Daleks to gestate in secret on the fringes of a populated galaxy like Antares. He accepts that Murgat has the best intentions, but he’s fought the Daleks long enough to know that they can’t change. Murgat pleads with him to have faith, but the Doctor wonders whether Murgat would be prepared to destroy his life’s work and wipe out the brotherhood if it began to fall apart. Murgat reveals that if the pollen should ever cease to work, the facility’s anti-matter reactors are booby trapped and all he has to do is say one word and the whole planetoid will be crushed to a singularity. The Doctor doubts that Murgat would be strong enough to do it, but Jesic assures him that she’d do it herself without a moment‘s hesitation. She offers to stay and say the word if need be. The Doctor said he was going to take her with him in the TARDIS back to New Davius via the scenic route, but although Jesic is tempted, she says it’s worth staying here if there’s the slightest chance Murgat might succeed. It would mean the war that scarred her and countless others would finally be over.

Reluctantly the Doctor agrees and he says goodbye to Jesic. Although he still thinks she’s made the wrong choice, at least it’s saved him from doing something terrible. He tells Murgat he still believes this is an impossible dream, but he wishes him well. The Doctor and Charley head for the TARDIS, but there’s one thing that’s still bothering him - how is it possible that the Daleks remembered Charley from Folkestone? Murgat suggests it’s possible these events haven’t happened for her yet and this seems to make sense. The Doctor recalls his own visit to Folkestone where he met Jessica Borthwick in October 1914 when she was taking Belgian refugees across the Channel in her yacht while under fire from the Germans. As the Doctor enters the TARDIS, Murgat takes Charley to one side and warns her she can’t lie to the Doctor forever. She promises that she’ll tell him when the time is right, but Murgat says he wouldn’t like to be in her shows when the Doctor discovers he’s been deceived. After the TARDIS departs, Murgat tells Jesic they have great work to do…

Much later, Jesic is screaming in agony as the Daleks torture her. Murgat pleads with the Red Dalek to let her go, but now that it’s clear the experiment has been a failure, Jesic tells him the Doctor was right all along. The new Dalek leader, the Red Dalek, accuses Murgat of corrupting them and holding them to a false doctrine, but Murgat insists that he’s freed them. Jesic begs Murgat to say the word and tells the Daleks they it’s been buried deep within them all this time. Murgat tries to persuade the Daleks to think of a bright future for their brotherhood, but the Daleks wish only to be reunited with their brothers on Skaro so that together they can spread the word across the Universe. And that word shall be…exterminate!!!! Murgat screams in despair as the Daleks take up the chant and the word is repeated throughout the facility.