Aboard the Presidential spacecraft Eschaton, Madam President issues a final warning to the approaching space vessel, but when the response is the same garbled message as before, she gives Captain Pargrave permission to open fire. He targets the unidentified vessel’s unstable warp engine and completely destroys it. Pargrave orders the crew to power down, but the President finds it amusing that he always thanks the Artificial Personnel too. She returns to her private suite and asks to be notified once they’ve located the Chaos Pool. The ships’ scientific team joins Pargrave on the bridge and he greets their leader, Zara.

The TARDIS materialises inside a spaceship and the Doctor and Amy are surprised to find the atmosphere is humid. The floor is drenched with some sort of mucus and the Doctor concludes that the crew probably use it to move around the ship. He can sense the warp engines straining and says they must be travelling extremely fast, but Amy can’t locate the final segment of the Key to Time anywhere. They hear some of the crew approaching and take cover, but find themselves trapped inside a cupboard.

There have now been five attacks on the Eschaton, but they’re happening more frequently and Pargrave is worried they won’t have time to fully recharge their weapons array. The President thinks their attackers are simply bandits looking for an opportunity to raid them, but Zara knows the Chaos Pool is nearby and says it’s more likely the aliens are defending it. Pargrave thinks it’s a bad omen, but the elderly Professor Lydall dismisses this comment as superstitious nonsense. As one of the APs helps the ageing man to a seat, he comments on how well Madam President is looking for a woman of considerable age, especially as all their friends and family died years ago. Nobody knows her secret, but as she’s rich enough to be able to fund this mission, Lydall suspects it’s down to endocrine surgery. Pargrave rebukes them for mocking his superior and leaves for his cabin. Once they’re alone, Zara complains to Lydall that Pargrave barely notices her, but the old scientist reminds her that the Captain is still grieving for his wife and children.

Through a small window, Amy observes the aliens in the corridor and says they look like giant slugs. The Doctor identifies them as Teuthoidians and they listen as Commander Hectocot and his underling Maddenjot discuss their search for the cause of a temporal anomaly. The Doctor says the Teuthoidians were among the Universe’s first space faring races, which suggests they’ve travelled a long way back in time. The slug creatures discover the TARDIS and conclude that it must be what they’re looking for. Hectocot orders Maddenjot to dispose of it, then crank up the ship’s engines. When Maddenjot questions his Commander’s orders, Hectocot becomes angry and bites off the creature’s head. He then orders the others to bring the ship out of hyperspace and prepare to attack, adding that they can eat the rest of Maddenjot once their mission is over.

Thinking the creatures have gone, the Doctor and Amy emerge from hiding - but they’re immediately captured by one of the slugs who remained behind to clean up the mess. Dismissing them as alien vermin, the creature is about to open fire when the entire corridor is shaken by a massive explosion as the ship’s engines are finally pushed too far. Fortunately their captor took the full force of the blast, but the fire is getting out of control, so the Doctor uses an extinguisher to put it out. Then, instead of returning to the TARDIS as Amy had hoped, he decides to investigate the engine. He soon discovers that a warp breach is imminent, which will destroy the whole ship unless he can stop it. Amy is afraid of dying and the Doctor realises how much she’s changed in the few weeks that she’s been “alive”. He tells her he’s afraid of dying too, but he’s even more afraid of not living. Despite the noise coming from the overheating engines, Amy hears a woman’s voice faintly calling her name and she wanders off to investigate…

Zara knocks on the door of Captain Pargrave’s private cabin, but he tells her to leave him alone. He hears a woman’s voice faintly calling him to join her in the Chaos Pool. The voice says he will find his destiny soon and reminds him he is the sixth son of his family’s sixth generation. It fades away when Zara enters the room, but just then news arrives that another alien attack is imminent. Pargrave orders the APs to be powered up and heads straight for the bridge. Zara is convinced she heard a woman’s voice coming from the cabin and believes it may belong to her sister, Amy. She tries to contact Amy telepathically, but there’s no response and she accepts that she must be getting paranoid. Then, just for a moment, she thinks she can hear Amy talking back to her…

On the Teuthoidian spaceship, Amy is about to tell the Doctor that someone was calling her when more of the huge slug creatures arrive outside the engine room. The explosion blew the door off its hinges, so there’s no way to keep them out, but the Doctor refuses to leave until his work is finished. Amy tries to stall the Teuthoidians and assures Commander Hectocot that she and the Doctor are trying to save their lives by fixing the damage. The Doctor is horrified when Hectocot reveals that he’s already ejected the ‘blue box’ into hyperspace. The Commander returns to the bridge to prepare for an attack and orders another underling, Jagrostab, to stay with the “alien vermin” and bring them to him once they’ve finished their work.

Pargrave arrives on the command deck and notes that the intervals between the attacks are getting shorter all the time. The unidentified alien vessel has emerged from hyperspace, but it’s evidently in a bad way and the warp engine is close to critical. As with all their previous encounters, the President plans to give them three warnings.

On the Teuthoidian spaceship, the President’s voice comes across the communications channel, but Commander Hectocot dismisses it as babbling nonsense. He orders his crew to ignore it and press on with the attack. He contacts Jagrostab in the engine room to find out how the repairs are coming along and the Doctor stops his work when he recognises the voice of Madam President! He listens as she explains that the Eschaton is on a mission of peaceful exploration. He tells Amy this is the voice of Romana. He knew she was working for the Time Lord Council, but he didn’t know she’d achieved the rank of President. He realises Hectocot can’t understand what she’s saying so he decides to make telepathic contact with Romana while Amy offers to go to Hectocot and translate for him.

Unfortunately Amy soon finds herself lost in the corridors of the Teuthoidian ship. To her surprise, she encounters someone she recognises straight away - her ‘sister’ Zara. But Zara attacks Amy and knocks her unconscious, then announces her intention to find the Doctor.

The Teuthoidians are now only a minute away from being in range to attack the Eschaton. Although he can’t understand a word Madam President is saying, Hectocot recognises her tone and dismisses it as being ludicrously pompous. He sends a message back telling them they are now the property of the Teuthoidian Empire and should prepare to be boarded.

Captain Pargrave, who is himself the sixth son of a sixth generation, notes that this is the sixth identical attack from the aliens. Madam President regards his comment as superstition - as far as she‘s concerned, they’re using exactly the same tactics as the other ships and will suffer exactly the same fate. They target the vessel’s unstable warp engine and prepare to destroy the ship…

The Doctor concentrates and tries to make contact with Romana, but to no avail. Just then, Amy cheerily returns and lets out a small shriek when she sees Jagrostab…as if she’s seeing him for the first time. Angry at his name-calling, she picks up a live power cable and thrusts it towards the huge slug. The Teuthoidian is killed instantly and the Doctor is horrified by what she’s done. To make things worse, the power surge has boosted the warp breach and there’s no way he can fix it now. The weapons array drops offline, making the ship completely defenceless. Madam President broadcasts another warning and the Doctor decides to patch the interior communications through the inter-ship link. He calls out to Romana and tells her he’s stuck on the enemy ship. When he adds that the weapons no longer work, Hectocot tries to switches off the communicator from his position on the bridge, but the controls are jammed.

The Doctor is surprised to find that he and Amy have been transported to safety aboard the Presidential vessel via some form of short range spatial displacement. He spots that Amy is wearing a Time Ring and asks her where she got it from, but she avoids answering the question. He tells her the person he thought was Romana is in fact the former Princess Astra, but that still doesn’t explain why she doesn’t recognise his name. Amy is amazed to learn that Astra was the disguised sixth segment of the Key to Time and that she was returned to her earlier state when he dispersed the Key. Amy insists they convert her back to the segment immediately and, ignoring the Doctor’s protests, she heads straight for the command deck.

Astra is furious when the Doctor and Amy burst onto the bridge. The APs grab hold of the two intruders and confiscate Amy’s Time Ring and satchel, and when they discover it contains devices with unknown power sources, Astra orders them to be handed to Professor Lydall for examination. The Doctor introduces himself to the President, but she still insists that she’s never seen him before. Pargrave believes they must have been stowaways here all the time and that they faked their message from the enemy ship. The Doctor explains that he was trying to repair the Teuthoidians’ warp breach when the Eschaton opened fire. Astra accuses him of aiding their enemies, but the Doctor reminds her that the Teuthoidian ship was crippled and had no defence shields. Suspecting that her memory must have been wiped when she was changed back from the segment, he asks her about Merak and the Marshal, but Pargrave says there‘s been no Marshal on Atrios for centuries. Pargrave suddenly realises that Amy is the spitting image of one of their crew members. She breaks free and grabs hold of Astra, intending to transform her back into the segment, but nothing happens and the President remains in her current form. Astra orders her to be removed and the APs drag Amy away.

Pargrave accompanies Amy as she’s escorted towards a guest suite where she can be held. Amy confirms that she is indeed their crew member Zara and reminds him of some of their earlier private conversations, including the fact that his children died from premature ageing because of the Atrion Curse and that he had to nurse his wife Francine through her final days. She then reveals that she loves him and says she can’t change back into her true form without the segments in her satchel, but then Pargrave hears a woman’s voice faintly telling him to bring the segments to the Chaos Pool. Believing ‘Amy’ must have been spying on him, he locks her inside the guest suite and leaves her banging on the door.

The real Amy is a prisoner inside the dimensions of the satchel, along with the Teuthoidians from the other ship. She can feel the segments nearby and knows they’re decaying. She hears the woman’s voice again and asks who it belongs to, but the woman tells her she will know soon enough - but by then it’ll be too late and the Key to Time will belong to her…

The Doctor apologises for his friend’s behaviour and explains that she’s not quite herself at the moment. He’s noticed that the Eschaton is “wobbling” and when he checks the control panel he discovers the cause is temporal fluctuation. Astra reveals that she’s over 200 years old and says she has no idea why she’s survived while her people have aged and decayed, but he suspects it’s a side effect of her release from the Key to Time. He’s also noticed that she has very few humanoids among her crew and learns that there are very few able-bodied Atrions left. Suddenly the command deck is filled with Teuthoidians who appear from nowhere. Astra calls for support, but the Doctor assures her there’s no danger as they’re simply temporal projections of something that’s yet to happen. He demonstrates this by walking right through one of the slug creatures. Convinced that she needs the Doctor’s help after all, Astra orders one of the APs to take him to Professor Lydall.

Lydall demonstrates his singularity matrix, but the Doctor’s extremely doubtful that Atrion science is capable of creating such a thing. He also points out that the Teuthoidians originate from the beginning of the Universe while the Atrions are much closer to the other end, and as neither race has discovered time travel, they shouldn’t exist at the same time. The Doctor tries to retrieve the satchel and segments from Lydall, but the elderly scientist threatens to have the AP break his arms unless he returns them. The Doctor warns him to keep them away from the singularity matrix, but he refuses to obey unless he tells him what he knows about the Chaos Pool.

Just then, Captain Pargrave announces another incoming attack and orders everyone to their stations. The Doctor thinks the singularity matrix is amplifying the effect of the Teuthoidian attack and when Lydall reveals that this is the sixth time the Teuthoidians have attacked, the Doctor realises there must be a fold in hyperspace, making the two extreme ends of the Universe co-exist. The possibilities are horrifying as there could be literally hundreds of Teuthoidian vessels on their way. It could even cause the Big Crunch, an unstoppable torrent as the Universe condenses to a singularity. Pargrave announces a red alert as more than a dozen Teuthoidian ships appear, but the Doctor realises their warp signatures are identical. These aren’t different ships, they’re multiple attacks from the same ship! In order to persuade Lydall to remove the segments from the singularity matrix, the Doctor reluctantly admits that the Chaos Pool really does exist but says it was hidden by its creators. He tries to grab the satchel back, but the AP drags him away.

The Doctor tells Astra that Lydall is responsible for the multiple attacks as his experiments have virtually ironed the hyperspace fold flat. It’s too late to send the Teuthoidians back, but they can at least stop more of them coming through. The ships are only 30 seconds away, so Pargrave orders the APs to target the nearest one and prepare to open fire. This time he refuses to give the attackers a warning and although the first ship is destroyed, the others continue to advance. Pargrave hears the woman’s voice inside his head again and she tells him the Chaos Pool awaits him. To everyone’s surprise, he gives the order for the crew to abandon ship. Astra points out that there’s nowhere for them to go, but Pargrave reveals there‘s a planet below which has until now remained unnoticed. The Teuthoidians latch on to the Eschaton and start cutting through the hull. Pargrave contacts Lydall in the laboratory and tells him the ship is about to be overrun. The Doctor asks where Amy is being kept and goes to release her, but Lydall insists that the satchel and its contents remain with him.

The Teuthoidians finish cutting through the hull and Commander Hectocot leads his troops, including Maddenjot, inside the Eschaton. He orders them to seize the vessel, slaughter the crew and bring the tastiest specimen to him…

As Astra heads for the life rafts she bemoans the fact that they’ve had to abandon their mission at the last hurdle. Suddenly the Teuthoidians appear at the end of the corridor and Pargrave leads the President to safety while the APs confront their attackers. The slug creatures start tearing the artificial lifeforms apart, but they evidently taste disgusting and Hectocot demands his troops find him real flesh, not mechanical devices. They hear the Doctor calling out Amy’s name and head off in his direction. The Doctor tracks Amy down and releases her from the guest suite. He believes the reason Astra didn’t change into the segment when Amy touched her is either because Amy has become more human than Tracer or because they’re right on top of a fold in space-time. Amy is horrified when the Doctor reveals that he had to leave the satchel behind, but before she can rebuke him, the Teuthoidians arrive. They run for their lives, and fortunately the Teuthoidians aren’t able to move very quickly on this ship as the corridors aren’t awash with their mucus. Amy seems confused by this and the Doctor tells her she should pay more attention.

Just then, another ship locks on to the Eschaton and moments later, a second version of Hectocot and Maddenjot cut their way through the hull. Before long there are so many ships attached to the Eschaton that it starts to have an effect on its orbit, causing it to lurch about. Astra and Pargrave encounter Lydall near the life rafts and the voice inside Pargrave’s head tells him the satchel contains the segments. Inside the other dimension, the real Amy can hear Astra as she demands to know where Zara is, but the voice inside Pargrave’s head tells him to forget the others and come down to the planet below. Obediently, Pargrave follows the instructions and orders Astra and Lydall to accompany him.

The two separate versions of Teuthoidians, led by Hectocot and Maddenjot, soon encounter each other in the corridors and as both groups believe the others are impostors, they start fighting amongst themselves. In the confusion, the Doctor and Amy manage to make their escape.

Astra and the others climb inside a life raft and close the door just as the Doctor and Amy appear at the end of the corridor. Pargrave refuses to wait any longer and the life raft is sent hurtling out into space, which Amy seems to take as a personal insult, but fortunately there’s another one she and the Doctor can use. The Teuthoidians watch as the two escape pods disappear into the darkness, but Hectocot assures his second-in-command they won’t get far.

As they descend down towards the planet, the Doctor and Amy look back and see that the Eschaton is now covered with hundreds of Teuthoidian ships. It evidently won’t hold orbit for much longer and there are more ships pouring through hyperspace all the time. The Doctor discovers the life raft is programmed to get the occupants to safety as quickly as possible, but he can’t steer it and the g-force will cause them to black out shortly…

Some time later, the Doctor tries to get Amy to wake up. They open the canopy to the life raft and find they’re in the middle of a graveyard of spaceships, some of which landed safely but many others smashed up on impact. It’s cold and wet outside, but the fuel line has ruptured, so they have to evacuate quickly. Moments later, the life raft explodes. They look for shelter and eventually stumble across the wreckage of the Eschaton which must have crash landed ahead of them. There’s no sign of any Teuthoidian ships, which is odd as they wouldn’t have had time to disentangle themselves. The Doctor guesses that once the Eschaton entered the atmosphere, the other ships were whisked off into hyperspace and back to their original place at the beginning of the Universe. The temporal barrier will have re-established itself and the planet will once again be camouflaged. The Doctor is surprised that Amy doesn’t recognise it. This is the planet where they first met, the planet Chaos where she was ‘born‘. They hear someone calling out and go to see if they can help. The creature is a bipedal canine and the Doctor hopes it can use its sense of smell to lead them to Pargrave and the others.

Professor Lydall and President Astra try to keep up with Captain Pargrave who’s marching ahead of them. They can still see the light from his torch and realise he must have already crossed the stream. They have to take things carefully because the rain is making the rocks slippery and as if to prove that point, Lydall accidentally drops his walking stick and it disappears through a crack and is swept away. They wonder whether they should stop for a rest, but they both feel strangely compelled to continue. When they catch up with Pargrave they find him standing beside a huge impressive mausoleum built into a rock wall. Above it are hundreds more, stretching so far into the distance that they can hardly see the top ones. There are carvings on the stones and Astra recognises the images as the creatures who attacked the Eschaton. They all feel as though they’re being drawn onwards, but Pargrave in particular seems to know in which direction they should be heading.

Inside the other dimension, Amy can sense that the segments are starting to decay and destroy everything around them. Lydall tells Astra that the satchel is starting to feel warm. He faintly hears Amy’s voice calling out for help, but Astra thinks the mausoleum is just giving him the creeps. She calls out to Pargrave, who’s gone ahead into the stone building, and asks if he has someone with him, but he just calls back and tells them they need to press on. They hear the noise of an animal and assume it must be sheltering from the storm, but Astra can’t help wondering if it’s tracking them.

The Doctor and the false Amy have lost sight of the dog creature that was leading them through the darkness, but there’s an archway ahead of them so they move through. They enter a cave system and are glad to be out of the rain, but there’s no sign of the animal. The Doctor reveals to his companion that he knows she’s an impostor and he holds up a mirror that shows her appearance has started to change back into that of Zara. He says he realised the truth some time ago, but he didn’t say anything in the hope that she might lead him to the real Amy. Zara reveals that Amy is inside the satchel and the Doctor is furious because he knows that could kill her.

Pargrave tells Astra and Lydall that the Chaos Pool is very close, but the elderly scientist is suffering from exhaustion and is on the verge of collapse. Astra thinks it’s odd that Pargrave seems to know the route, even though they’ve never been here before. Lydall gets an electric shock from the satchel and burns his hand badly. At first Astra tells him to leave it behind, but then she and Pargrave hear a woman’s voice inside their heads telling them to bring the satchel to her. Lydall tries to open it first and discovers the satchel is falling apart…

Zara confesses that she took Amy’s form in the hope that the Doctor would lead her straight to the next segment - but now that she’s touched Astra and it had no effect, she realises she’s become so human she’s no longer able to function as a Tracer. The thought that the other segments might now be lying beneath the crushed wreckage of the Eschaton makes Zara cry and the Doctor gives her a hug to comfort her. He asks her about Pargrave and she tells him they met on Atrios after she discovered he was the sixth son of the sixth generation of his family. She was going to use him just the way she used Zinc back on Mars. She knew he would eventually be drawn to the Chaos Pool just like the other pilots of the crashed spaceships. She looked after him for months after his partner and children died, but she never expected to fall in love with him. Pargrave doesn’t know how she feels because he’s only interested in the Chaos Pool. Just then, Zara hears Amy’s voice calling for help and she tells the Doctor that the satchel and the segments are disintegrating. They also hear the dog creature coming back in their direction, so they try to move away before it gets their scent…but it’s too late! The Doctor politely introduces himself and Zara to the dog creature, who then starts beckoning at them to follow him.

Pargrave, Astra and Lydall arrive at a set of huge doors. Fortunately, they’re slightly ajar and it’s possible to slip inside. They find hundreds of people standing in an ante-chamber before a woman with a face like a mask. There’s a waterfall with the water pouring straight into a large pool below, but although it gives off a reflection, there‘s no sign of any splashing as the water hits it. The woman looks over to them and calls for President Astra to join them. Lydall is surprised that the people know who they are, but both Astra and Pargrave seem only too pleased to be invited inside. As Pargrave looks down into the Chaos Pool in awe, the woman welcomes them and asks the third member of their party to come out of hiding. He nervously joins the others, but the woman suddenly demands to know who he is. As Pargrave snatches the satchel from him, Lydall pleads with Astra to leave with him, but she too insists that he hands over the segments. The woman tells him there are over a thousand freedom fighters here and says he has no choice, but Lydall challenges them and notes with some surprise that the crowd cowers away from him. He realises that they’re scared of whatever he’s carrying in the satchel, so using it as a form of protection, he turns and flees from the ante-chamber. Pargrave volunteers to go after him, but the woman tells him their true master is due to arrive here shortly.

Zara is worried that they’re following the dog despite knowing nothing at all about it. She thinks the Doctor is very trusting, but he’s surprised that she regards that as a bad thing. She remembers him offering to help the creature and can’t work out whether he’s brave or just foolhardy. As they move through the tunnels, Zara stumbles across Lydall’s walking stick and they realise it must have been washed downstream. Zara urges them to hurry in case the Professor needs their help, but just as the Doctor is about to congratulate her for thinking of someone else for a change, she points out that Lydall still has the segments in his satchel. They emerge from the tunnel and find themselves in the huge graveyard. The Doctor discovers the gravestone of Commander Hectocot and realises this must be the last resting place of the Teuthoidians. Some of the stones start coming loose as the statues begin breaking away from the wall itself. Hectocot himself emerges and immediately attacks the dog creature, but Zara holds the Doctor back to prevent him from attempting a rescue. More Teuthoidians, including Maddenjot, emerge from within the stone walls and the Doctor and Zara have no choice but to abandon their new travelling companion and run for their lives.

As they flee, the Doctor points out that the planet Chaos is being held in stasis just a few minutes away from the end of the Universe, but the Teuthoidians come from the other end. He realises they’re right on the fold in time, where the beginning and the end of the Universe are able to co-exist side by side. As they enter a tunnel, they see Lydall ahead of them, still carrying the satchel. He hands over Zara’s belongings, including her bracelet, and they notice how badly burned he is from contact with the segments. Zara opens what remains of the satchel, but there’s no sign of the segments inside. Instead, to the Doctor’s delight, Amy appears. Before they can discuss the matter further, the Teuthoidians arrive at the end of the tunnel so they decide to move out quickly.

Out of earshot of the others, Amy tells the Doctor that the segments were starting to disintegrate so she placed them instead inside her own satchel to stop them harming her while she was trapped with them. The group arrives at a set of huge doors - but as they enter the ante-chamber, hundreds of people turn to look at them. The woman at the head of the groups welcomes them - particularly Amy - and says she can sense they’ve brought the segments with them. The Doctor notices her followers are made up of many different races from across the Universe. The woman describes them as freedom fighters, an army set up by her masters to defy the power of the Key to Time. With the Teuthoidians close behind then, the Doctor asks for help from the crowd in closing the heavy doors, but to his amazement, the frail elderly Professor Lydall is able to do it alone and unaided. The woman starts to protest, but Lydall commands her to be quiet and both she and the crowd are frozen to the spot. The Doctor realises the truth - Professor Lydall is none other than the White Guardian.

The Guardian admits the truth, but assures the Doctor that he didn’t know his true identity until a moment ago when he was revitalised by exposure to the segments. After the Doctor abandoned him back in the 9th century, Zara rescued him and nursed him through the years as they hunted for the Chaos Pool. The Guardian asks Amy to hand over the segments, but the Doctor tells her to keep them safe. He points out that if the White Guardian gets the Key there will no longer be any disorder or free will in the Universe. He isn’t a Guardian of virtue, but a Guardian of control and domination, someone who follows the letter of the law but not the spirit of it. The White Guardian argues that chaos has no boundaries and says people can’t just make up the rules as they go along, but the Doctor says that having absolute certainty about everything is just another form of tyranny. Amy backs up the Doctor and when the Guardian accuses him of putting thoughts into her mind, she points out that he’s a teacher, not a dictator, and she’s able to make her own choices.

The Doctor is dumbfounded by this revelation. Romana explains that after Astra was returned to Atrios, the segment was hidden within her. She didn’t realise at first, but this is why her first incarnation decided to change her appearance. The woman in the crowd insists that the Key to Time must not be assembled, but just then there’s a loud banging on the door and the White Guardian reveals that his own servants, the Teuthoidians, have arrived. The two Guardians begin to squabble as the White Guardian orders Zara to release the segments, but the Black Guardian orders the woman from the crowd to stop her so that the segments can continue decaying. The Doctor urges the woman to resist being treated like a slave and shows her how much the two Tracers, Zara and Amy, have learned about themselves in the brief time they’ve been alive. But although the woman has lived here for generations building up her army of freedom fighters, in all this time she’s learned nothing from being with them.

There seems to be an impasse as the Doctor and Amy struggle to protect Romana from the White Guardian and Zara, while the Black Guardian orders the woman and the freedom fighters to advance. Amy realises she doesn’t have to obey either of the two Guardians and in a demonstration of resistance, she reaches out to Romana and touches her. Romana is enveloped in a flashing light, but the Doctor races forward and drags Amy away. As Amy flees from the ante-chamber, Romana collapses to the floor, but she remains in her original form and appears relatively unscathed,. However, she reveals to the Doctor that her body was deteriorating anyway. She was diagnosed by the medics on Gallifrey when she returned from E-Space and was told that regeneration would be impossible. The Doctor tells Romana this castle is the former home of the Valdigian known as Lady Mesca, which means they’re on Safeplace. The castle itself is one of the segments of the Key to Time, which explains why the building is in such a state of decay. Amy must have taken the segment from the satchel dimension and changed it back, but the transformation can’t be reversed while the other segments are inside the building as it would be too dangerous to have one segment inside another. The Doctor says they need to find Amy…

The recently revived freedom fighters, including President Astra and Captain Pargrave, are ordered by the nameless woman to defend the Chaos Pool and hold back the intruders, but it’s not long before they find themselves being overrun by the Teuthoidians. Zara pleads with the two Guardians to help and reluctantly they both agree to stop their supporters. In an instant, everyone else in the ante-chamber apart from Zara and Astra are frozen in time again. Zara suggests converting the castle back into the segment, but the White Guardian warns her that would destroy the Universe unless the other five segments are retrieved first. When she uses her Time Ring to go after Amy, the Black Guardian accuses her of breaking the rules, so he decides to revive his freedom fighters. In retaliation, the White Guardian revives the Teuthoidians…and the battle resumes!

The Doctor and Romana are getting reacquainted when they hear the Teuthoidians smash their way into the castle, so they race upstairs. Moments later, Hectocot and Maddenjot explore the lower level then send their slug-like troops on a mission to wipe out any vermin they encounter. Not far away, Amy is about to use her abilities to trace Romana when she hears Pargrave and his crew searching for the enemy intruders. She hides in the cell where she was once tortured by Mesca - but she’s not alone…

While their respective troops ransack the castle, the two Guardians are casually engaged in a game of bridge with their partners, Astra and the unnamed woman. The game is a perfect example of the differences between the two and they both taunt each other, claiming their approach is the better.

The Doctor and Romana arrive at the castle’s monitoring room, but unfortunately all the equipment has rusted into scrap. The Doctor says the return of Astra to Atrios came at a terrible price, not just to her but also to her people. She survived for 200 years by drawing energy from those around her, living forever as everyone else became extinct, with no idea that she herself was the cause. Romana points out that she too is just a shell of her former self and she knows the Doctor has already worked out that they have no choice but to complete the Key to Time by converting her into the sixth segment.

In the cell, one of Pargrave’s officers tries to retrieve Amy of her satchel, arguing that his mistress wants it destroyed so that the segments can be allowed to decay. Suddenly Zara appears from nowhere and tries to snatch the satchel from their grasp. Then, Pargrave himself arrives and tries to take it for himself. As the four of them struggle over the satchel, it finally rips apart and the segments fall to the floor. Amy and Zara manage to grab two of them each, but Pargrave threatens to kill them unless they hand them over to him. Zara appeals to him and reminds him that she stood by him after his family died, but he tells her she’s dead to him too. Zara realises that one of the segments she’s holding was once the lake from the planet Erratoon. Warning Amy to hold on, she transforms it back into its original state and the entire room is suddenly filled with water.

The Doctor and Romana are looking for an exit when Maddenjot and his Teuthoidian troopers appear and block their escape. The slug creature contacts Commander Hectocot with the good news - but then the entire area is suddenly filled with water which pours down the corridor, sweeping everything along in its path. The Doctor and Romana manage to reach a staircase and climb above the water level, but it’s rising incredibly fast and before the Doctor can reach out to save her, Romana too is swept away…

Eventually the water level settles down and then moments later it disappears completely, leaving the already crumbling interior of the castle in ruins. The Doctor tracks down Romana and he notices immediately that her clothes aren’t wet. Zara and Amy join them and tell him they have four of the segments and have just returned the lake of Erratoon to its crystalline state. The Doctor wants to get Romana to the TARDIS, but she and the two Tracers insist on going to the Chaos Pool instead. It’s where Zara and Amy were born and where the Key to Time was originally forged. The Pool is a portal to the higher dimensions where the Grace will be waiting to restore balance. Romana realises the Key isn’t the solution, it’s the problem, and it needs to be reassembled or annihilated in order to save the Universe. The Doctor argues, but the others all agree they have no choice and Romana has already made up her mind. The Teuthoidians arrive again, so the Doctor leads the others across the banqueting hall - but they run straight into the hands of Captain Pargrave and the other freedom fighters…

Astra questions the unnamed woman about her identity and suggests that she prove to her master, the Black Guardian, that she’s free to choose a name for herself. The two Guardians are continuing their card game, aware that they can wait forever if they need to. The woman accuses Astra of being responsible for her people’s demise when she surrendered the essence of the sixth segment to Romana. Although Astra has lived for two centuries, the average life expectancy for people on Atrios is just 29 years. She reveals that Astra is drawing the life directly from the people around her and points out that despite all her efforts, she hasn’t been able to save a single person.

The Doctor, Romana, Amy and Zara are now caught between the Teuthoidians on one side and the freedom fighters on the other. Amy suggests they use the lake of Erratoon again, but Romana says this would probably drown them all. Zara hands her Time Ring to the Doctor who tells everyone to hold hands. He activates the Ring and the group disappears into thin air. Commander Hectocot and Captain Pargrave confront each other in the centre of the room, baffled by what happened to their prey, then both groups turn on each other and re-start their fight to the death…

The two Guardians continue their card game and the result so far show they’re evenly matched. But the Black Guardian warns his opponent that the rules of the game are about to change…

The Doctor and the others reappear outside the castle, which is now on the verge of collapse. Zara tells Amy to convert it quickly and she reaches out to touch the stone wall. There’s a flash of light and the castle disappears, to be replaced with a small crystalline segment. The two Tracers fit the five pieces together, leaving a space for the final sixth segment. Just then, Romana collapses. The Doctor rushes to help her and she reminds him that she’s dying. The two Guardians join the group and congratulate Amy and Zara on their success, adding that this now means their life‘s work is over. The White Guardian summons Astra and orders her to bring him the Key, but she refuses. The Black Guardian orders the unnamed woman to do the same, but she announces that her name is Freedom and she is free of his control at last. The Doctor realises that because the two Guardians are evenly matched, they both need the help of someone else in order to give them the edge.

Astra approaches Romana on the ground and gives her a hug. In that instance, the essence of the sixth segment leaves Romana’s body and enters Astra instead. Romana appears to be fully recovered and gets to her feet, explaining that Astra chose to sacrifice herself to save her. The sixth segment joins the others and the Key to Time is now complete, but it will remain inactive until Zara and Amy let go of it. The two Guardians immediately demand that their respective Tracers hand the Key to them, but both women refuse to move. Suddenly the Grace appear, manifested inside the Key itself. They declare that neither of the Guardians should have the Key and they banish them both back into the howling void so they can resume their eternal struggle.

The Doctor congratulates the Grace on their impressive display of power. They reveal that they made Freedom to be their servant throughout eternity, but she no longer wishes that, so they offer the only freedom left to her. In a flash, she too disappears with a scream. The Doctor is furious, but the Grace ignore him and order the two Tracers to release the Key to Time and surrender it to them. The Doctor points out that Amy and Zara aren’t Tracers any more, but human beings with all the strengths, weaknesses, foibles and marvels that come with that. The Grace laugh and say the women have no choice but to do as they demand.

The Doctor asks Amy if she trusts him and when she says she does, he tells her to let go of the Key. Then he tells Zara she has the choice of either surrendering the Key to the Grace or giving it to him. The Grace threaten to crush the Doctor, but he knows that they’ve manifested themselves inside the Key and they can’t leave it until everyone has let go. Zara asks Amy what she should do, but Amy insists that she makes the choice herself. Zara is angry that the Grace refuse to address her by her name, so she hands the Key to the Doctor, then he turns and jumps straight into the Chaos Pool. There’s a flash and everyone peers over the edge to see that the Pool has turned into a dark chasm full of jagged rocks. Moments later, the Doctor calls up to his friends and asks for their help getting him out.

The Doctor admits that he had no idea there was a ledge just beneath the surface of the Pool. All he knew was that it would disintegrate the Key and disperse the Grace throughout eternity. Everyone who was trapped in the castle segment will be returned home, including Pargrave. He hands the Time Ring back to Zara and invites her to join the man she loves back on Atrios. She’s surprised that he’s willing to let her go after everything she’s done, but he reminds her she was also brave and compassionate. He tells her nobody is perfect and that’s what separates real people like her from artificial lifeforms. The Doctor took a risk giving her the choice, but if he’d compelled her then he’d have been no better than the Grace. He risked everything on his faith that Zara was really a good person. Zara begins to cry. She invites Amy to come with her, but Amy says she has other plans. Zara thanks the Doctor and says goodbye to everyone, then she operates the Time Ring and heads for Atrios.