The Doctor has lost Peri on an unnamed planet where the rain never stops. He calls out her name, but the thunder and wind prevents his words from travelling far. He examines a compass he discovered inside the wreckage of a spaceship called ‘The Green Man‘s Burden‘, but its readings are unclear. He suddenly realises he’s not alone and greets a beautiful young woman who appears from nowhere. He thanks her for whatever she did that stopped the rain, but she points out it’s time itself that’s stopped. He can tell she’s not a survivor from a crashed spaceship, but she claims not to have a name and says she was built by the Grace. The Doctor has never heard of them, but if they’re able to stop time he deduces that they’re probably huge and pan-dimensional and have amazing powers. The woman explains that the Grace want her to track down three segments of the Key to Time and she’s come here to ask the Doctor to be her assistant.

The woman explains that the Key to Time was also built by the Grace. It’s a perfect cube which maintains the equilibrium of time itself. When its six crystal segments are assembled, it can stop and start the Universe, re-write matter and change the states of quanta to restore balance. The Doctor already knows the six segments are scattered throughout time and space, disguised as anything or anyone. The Doctor refuses to help and says he’s busy doing lots of things, but the woman says time will remain stopped on this planet until they get back from their mission. He realises now that the Grace must have brought the TARDIS here and it was the compass from the ship that somehow led him to the woman.

He asks her to prove she doesn’t work for the Black Guardian or want the Key to Time for her own reasons, but the question confuses her. When he hands her the compass it immediately changes into one of the crystal segments and he realises the woman is actually a Tracer. She says her nose tickles in the presence of the segment. She places the segment in a satchel and says the inside is in a different dimension and it has powers this Universe can barely conceive. The woman seems quite naïve about the dangers she’s likely to face and she admits that she’s only four minutes old. She tells him the Grace didn’t want to involve him as they find him dangerous and unpredictable, but that‘s precisely why she thinks he’ll be useful. He agrees to help her find one more segment, but then she can find the third segment herself, leaving him free to come back here to find Peri. As they head for the TARDIS, the Doctor asks if she has a name. She says she doesn’t so he decides to help her choose one. He starts with the letter A and tells her to stop him when she hears one she likes…

Instead of setting co-ordinates, the woman takes control of the TARDIS. The Doctor continues listing names and by the time the ship materialises at its destination, he’s reached ’Amy’. The woman thinks the landscape they’ve arrived at is amazing, but the Doctor mistakes her reaction and thinks she’s settled on that name. The temperature here is warm, which is a new experience for Amy, and the area is surrounded by pools and fountains which act as a central cooling system. They’re close to a fortified town and judging by the state of the place, the people here have been having a fiesta. He asks her where the second segment is and she allows her tickly nose to lead them.

They duck down as a figure appears, but it’s too late and they’re spotted. The Doctor insists on doing the talking and introduces Amy to the Ice Warrior. The Martian recognises them as aliens and declares there are rules for dealing with such situations - but to the Doctor’s surprise, this means the creature is obliged to offer them some of his water. The Martian doesn’t recognise the word ’warrior’, but when he recommends they visit the nearby marketplace, the Doctor is amazed to discover they’re on Mars. He’s only ever been here when the Martians were long gone and is surprised to find the place so peaceful. Oddly, for a species that can’t abide heat, they’ve built a town right on the equator, but the shell-like buildings obviously reflect the heat away and the canals and waterways help to keep the air cool. Not far away they can see a huge Pyramid under construction and Amy’s senses tell her the second segment is the capstone on the top. It‘s not going to be easy clambering up there and taking it in full view of everyone.

Suddenly the entire area is rocked by an earthquake. The people in the market panic as the buildings collapse around them, and then everything goes black. Once things settle back to normal, the Doctor and Amy start organising the survivors into parties to dig for people buried under the rubble. Six Martians are dead, but many others are badly wounded and are suffering from cracked shells and blood loss. The Doctor arranges for them to be taken on stretchers to the apothecary. When he meets up with Amy again, he notices she’s not injured at all and isn’t even bruised. They both found that the Martians kept offering them water and the Doctor says they have a gift economy here, so people don’t pay for things, they’re given them free under a strict code of honour that requires the receiver to offer something in return. This isn’t what the Doctor was expecting - these Martians are behaving more like an arts and crafts commune than soldiers. They hunt for food, but they don’t fight each other and they’ve never experienced warfare.

A Martian places them both under arrest and they’re taken to be questioned by Isskar, an elderly Martian magistrate who tells two aliens, a male and female, are operating in the area as thieves and although the Doctor tries to convince him that not all aliens are the same, he understands why they’re under suspicion. Isskar tells him his people will keep a close watch on them from now on. The Doctor thinks the Pyramid might be the cause of the earthquakes, but the magistrate tells them it was built by 19,000 masons, 600 carpenters and 46 overseers and their engineers would not allow for any errors. He admits there have been earthquakes before, but although their rate has been increasing in the last few days, their strength has been decreasing. Isskar demands that they both stay away from the Pyramid and warns them that trespassers will be punished severely. Ignoring Amy’s protests, the Doctor apologises for wasting so much of Isskar’s time and suggests they both return to the market to look for postcards.

Amy is surprised that Isskar let them go, but the Doctor assures her the Martians are reasonable people. Just then the ground starts to shake again and Amy predicts another earthquake. They head for the entrance to the building site at the base of the Pyramid and break in. Inside, Amy’s nose starts to tickle, but this time it feels different and she thinks it means there’s someone in here with them. They hide as a young humanoid woman enters behind them. Suddenly a Martian grabs the woman and accuses her of being a trespasser. She protests that she’s a tourist, but he drags her away and Amy watches as the woman is reunited outside with a young human man. They claim to be innocently taking photographs, but the Martian refuses to listen. Amy leaps to their defence and demands that they be released, but while the creature is distracted, the man smashes a rock over him and he falls to the ground, dead.

The Doctor arrives just as the woman is congratulating her partner for his swift action. It’s obvious these two must be the alien thieves Isskar mistook the Doctor and Amy for. The man pulls out a gun and holds it at Amy’s head, but to everyone’s surprise the woman laughs when she hears Amy’s name and it becomes clear the two women know each other. The man refers to them as sisters due to their strong resemblance (and the fact that they‘re both carrying identical satchels) and the Doctor realises they must be after the other three segments of the Key to Time. The woman insists that because Amy already has one segment, it’s only fair they let her collect the one on Mars. None of them want to be discovered near the dead Martian, so they team up and leave the area together.

The Doctor, Amy and the other man and woman start climbing the scaffolding around the base of the Pyramid, fully aware that once they leave its cover, they’ll be visible to everyone on the ground. The man introduces himself as Harmonius 14 Zinc and the Doctor recognises his designation as belonging to the Commune of the White Sun. He spent a week there himself enjoying their tree-houses only to be imprisoned and threatened with torture for blocking out their sunlight. Amy thinks she can see parts of Zinc in the other woman’s eyes and in the way she moves. She suggests the woman should take a name too, but Zinc scoffs at the idea. The Doctor says it will help her to build an identity and become someone real, so he decides to list all the names again, starting with A. When Amy protests, he starts at the other end and eventually comes up with Zara, meaning Princess, which just happens to be Zinc’s mother’s name. The woman agrees to take it, if only to shut everyone up.

With 64 steps to go, the group sees a Martian climbing after them. He’s surprisingly fast, which the Doctor assumes is due to the low gravity being his natural environment. Zinc whips out his gun and the Doctor casually asks if he can see it, but then he deliberately lets it slip from his grasp and it falls to the ground. Zara and Zinc are furious, but the Doctor says enough people have died already. He asks whether they’ve worked out an escape plan and Zinc proudly produces a time ring that he picked up on Barampa where it was kept in a museum in the belief that it was purely ceremonial. The Doctor wonders why he doesn’t just use it to go straight to the capstone at the top and he explains that they’ve tried that already, but something keeps skewing the co-ordinates. The Doctor suggests that after they get the segment, they use the time ring to return to the market where his own transportation is waiting.

The Martian arrives at the top of the Pyramid and prepares to shoot. The Doctor points out that removing the segment will be like pulling a plug and they’re standing right over the hole. Zara and Amy can feel the intense pressure coming from the segment - but when they reach out to touch it, it’s like their fingers are on fire. Suddenly the earthquakes stop and Zinc wonders if there’ll be a reward. Zara shows them the crystal segment and says it’s keeping the black hole at bay. The Doctor tells her she can’t leave with it until he’s taken it back to the TARDIS and repaired it. Zinc inadvertently reveals that Zara now has two segments and she’s forced to admit that she lied earlier and she’d already found another segment on Erratoon. The Martian demands that she hand the crystal over and when she refuses he opens fire, but the sonic blaster has no effect as the segment is protecting her.

Zara is tired of Zinc constantly talking about how much the segment is worth and realises she’s outgrown him. She snatches the time ring from him and, despite the Doctor’s protests that she’s condemning them all, she operates it and disappears, taking the segment with her. The Pyramid immediately starts to collapse around them…

The Doctor, Amy and Zinc are amazed when a flying ship, commanded by the magistrate Isskar, appears in the sky right beside them. He extends the force field to catch them in mid-air and tells them they‘re under arrest, then the three of them leap across just as the stone building crashes to the ground. The ship is still caught up in the gravity well and they’re in danger of being torn apart so the Doctor advises them to fly straight into it rather than try to resist its pull. Isskar eventually agrees to let him take the helm and he skilfully pilots the ship to safety in a holding pattern about 200 metres above the ground which is ripping itself to pieces below them. They’ve got enough fuel to stay in the air for about another hour, but after that things are going to get tricky again.

Isskar accuses them all of making an unprovoked attack on his people, but the Doctor says its too late for that. They have a black hole on their doorstep and the canals are already boiling away due to the heat trying to equalise. The only thing they can do is evacuate the town, but the Martian is furious with them and plans horrible deaths in revenge. A message comes through from the Ice Lords ordering all ships in the area to save the people on the ground, so Isskar gives instructions for the ship to be taken down. Once they’re alone, the Doctor tells the others they need to escape and get to the TARDIS. Amy wants him to “heal” the black hole, but he says there’s nothing he can do and many of the Martians are going to die. The Doctor distracts the Martian guard and then wrestles him to the ground before he can fire his sonic blaster.

Down on the ground, Isskar supervises the evacuation of the survivors and assures them there’ll be room for everyone aboard his ship. News arrives that the alien prisoners have escaped, but then Zinc arrives and claims to have been an innocent bystander all along. Although the guard says he was recaptured, Zinc insists he surrendered voluntarily. He then betrays the Doctor and Amy by telling Isskar where their ship is, but Isskar announces that his reward will be a less arduous death. Zinc protests and begs for his life, but the magistrate mercilessly cuts him down with his sonic blaster.

The Doctor and Amy flee through what’s left of the marketplace, relieved that everyone seems to be too busy to chase after them. Amy doesn’t want to leave and says they should stay and help, but the Doctor knows it won’t do any good. With the Martians close behind them and firing at will, the Doctor reluctantly has no choice but to open the TARDIS and bundle Amy inside.

The Doctor apologises for the bumpy take-off and explains that the TARDIS had to travel right through the gravity well. This also explains why Zinc’s time ring kept getting knocked off course. Amy realises Zara knew perfectly well what would happen when she removed the segment. The gravity well will work itself out in time, but it’ll change the conditions on Mars permanently and the population will have to move somewhere new. Unfortunately the Martians are blinded by pride and millions of them will die before they’ll admit there’s a problem. The Doctor detects some spatial disturbances and suspects the other segments are eating holes in time itself. He’s angry that Amy didn’t warn him earlier, but she tells him it was his own fault for putting the Key to Time together without the proper sixth segment. The six segments depend on one another and the Grace never thought anyone would be so reckless as to assemble it with a substitute segment. It’s because the Doctor caused this in the first place that Amy thought it would balance things out if he helped her restore the Key. All the places where the remaining segments are hidden are in immediate danger. The planet of rain where they first met is the furthest location in the future and is only 66 minutes from the end of time. The Grace have finite powers and they can’t stop time there forever.

The TARDIS materialises in the corridor of what appears to be a castle. The Doctor is keen to grab the segment and leave, but Amy is having trouble locating it as she’s tickling all over. The Doctor says they’re on a world known as Safeplace and it’s been 16,000 years since they left Mars. The last time he was in the area the whole galaxy was at war, but an armistice was signed several generations ago. This district is inhabited by the Valdigians, one of the more civilised insect-like species. They wander around in the hope of picking up a stronger signal, but their path is suddenly blocked by the arrival of several winged guards who arrest them both on a charge of being “poisonous heretics“.

In the throne room, Thetris, Captain of the Ceremonial Guard, is desperately trying to treat the dying Lady Mesca, who he believes has been cursed by a rival faction led by Wembik. Mesca is becoming weaker by the moment, but Wembik insists her group is just as horrified by what’s happened and implies Mesca may just be unwell from the stress of the current proceedings. Mesca pleads with Thetris not to start a war. When the Doctor and Amy are brought in, Wembik immediately accuses them of being alien freaks and war-makers. The Doctor starts to explain that they’re on a sort of treasure hunt when he notices how ill Mesca is looking. He offers to help, but Wembik believes it’s a trick and demands the right to take Amy away to be tortured. Thetris agrees and, despite the Doctor’s protests, Amy is dragged away for interrogation. Then the Captain orders the Doctor to apply a counter-charm to remove whatever curse he placed on Mesca.

The Valdigian guards take Amy to a dungeon and Wembik prepares her instruments of torture. The insect woman admits that she doesn’t know anything about Amy’s biology, but says it’s usually a good idea to start with the subject’s extremities and then work inwards. Amy insists that she doesn’t know anything of value, but Wembik is certain she can’t remain silent for long. She demands to know how Amy cursed Lady Mesca and when she doesn’t get an answer she applies a heated blade to Amy’s arm. She’s surprised when Amy doesn’t react, but this only makes her more determined to make her prisoner scream.

As the Doctor examines the Lady Mesca, she asks him to make sure things don’t escalate into a war. Thetris reveals that Mesca’s son is the only rival to Wembik’s son as contenders to become King. They’re here to resolve the dispute and the castle has been designated a neural area within Safeplace, but the moment Mesca arrived she was struck down with a mysterious illness. Thetris accuses Wembik of having dark powers and the Doctor wonders whether she owns any unusual artefacts that could be the source of her magic. Thetris and Mesca were due to be married after the Kingship was agreed, but this fact has been kept secret to avoid confusing the matter. Thetris is determined that Wembik’s faction won’t win and says he’s prepared to go to war if he has to. The Doctor reassures him that Mesca will be fine - she has a bruise on her dorsal region with two tiny pinpricks in the centre. It appears to be a snake bite, but fortunately the Doctor has just the thing to cure it…

Wembik’s torture of Amy is not going well and Amy is almost starting to feel sorry for her inquisitor. Nothing she does seems to have any effect and she hasn’t even raised Amy’s heart rate! Wembik admits that she’s never done anything like this before and it goes against her nature, but she yearns for power and the chance for her son to be King is too strong a temptation to resist. Wembik says they’re descendents from refugees who survived the war, but everything is plentiful here and they don’t really need a King any more. Their laws state that a King can only rule from the age of 23 and until then a provisional government keeps the peace. As the people don’t want to be ruled, they elect a child who agrees to stand down at the age of 22 in return for a generous pension. This way they never actually have a King in power and they only have as much government as is absolutely necessary. They’re going to war over an entirely meaningless role and Wembik doesn’t even care if her son never becomes King - so long as Mesca’s son doesn’t either. The door bursts open and Thetris enters, demanding that she releases her prisoner. As Amy is released, she notices that her arm is genuinely hurting and a bruise is starting to form…

The Doctor is delighted when Amy is returned, apparently unharmed, but she shows him her bruise and he realises she’s becoming more susceptible to pain. His cure for Mesca is an anti-toxin made of snake venom taken from Mesca’s wound, but Thetris has insisted they try it first before administering it to his wife-to-be. The Doctor had promised it wouldn’t harm Amy, but now he’s not so sure. Amy readily volunteers, but the Doctor refuses now that he’s learnt she’s physically changing. Amy says she trusts him, so she snatches the anti-toxin from his hand and drinks it. She starts to feel weird and has to sit down. Her body trembles and the pain becomes intense, but then she starts to recover and assures everyone that she‘s alright.

Thetris insists that they give the anti-toxin to Mesca, but the Doctor warns that it won’t work as quickly on her. Mesca drinks the fluid and it’s not long before she starts looking much healthier. Thetris is delighted and agrees to let the Doctor and Amy go free, once Mesca has recovered fully. Just then, they receive news that several spaceships are heading straight for the castle and refusing to respond to any communications. Thetris tells the officers to stand down their defences and assures them their visitors are expected. He’s confident the ships belong to pilgrims and says Zara would not lie to him…

The Doctor starts examining the paintings on the wall, in case one of them is the treasure they’re looking for. Wembik once again accuses them of being responsible for poisoning Mesca - but Amy points out that Wembik herself was the only one who would profit from Mesca’s death. When Thetris recalls that Wembik once wrote a monograph on handling snakes, the woman is forced to admit that she was responsible. Amy tries to persuade Thetris there’s no need for a war now, but as the argument starts to escalate, the Doctor grabs everyone’s attention and tells them he just heard the sound of a spaceship coming in to land. Thetris says they are pilgrims and Mesca announces that all such people are welcome to Safeplace.

For 12,000 years, life on Mars prospered peacefully. It was a world of builders, craftsmen and farmers - until the day the ground and sky betrayed them. When the plug was pulled from the gravity well it sent earthquakes and hurricanes across the planet surface for three decades. Almost everything was lost. A few managed to get off the world, but the rest fought amongst themselves for food and shelter. After 30 years the energies were spent and the ground settled down. What remained of the Martian people emerged and planned the re-building of their world, even if it took them a thousand years.

But one Ice Lord, Izdaal, had been watching the sky instead. He addressed his government and told them they were wrong to think their turmoil was over. He’d studied the evidence and seen the sickening children and believed life on Mars was no longer sustainable. Their atmosphere was no longer capable of keeping out the malign radiation and it would eventually kill them all. His words were ignored by his people, so to prove he was right, he elected to step outside and face the Red Dawn, knowing it would be fatal.

Ice Lord Isskar recounts the story of his people as he wants the Doctor and Amy to know what they’ve done. He was a witness to events and saw the years of pain. He saw Lord Izdaal step out and die. The Doctor says he’s been to Izdaal’s tomb, but Isskar believes he’s being mocked and clubs the Doctor to the ground. He accuses the Doctor of destroying his world and murdering his family. His race are now no more than parasites, resorting to taking things from others that were not freely given. What happened on Mars was thousands of years ago, but the Martians will never forget it. He regards the Valdigians as his enemies for harbouring the Doctor and Amy, but the Doctor argues the insect race were once refugees too.

Isskar has spent millennia in suspended animation travelling thousands of light years just to track them down, but it’s impossible to believe he just stumbled across them by accident. Isskar admits that he had assistance and the Doctor realises he means Zara. He says the Martians have been used by the very person who was to blame for what happened. He questions the logic of Isskar’s justice and says it’s too late for them to lay down their arms now and go back to being builders. He says they’ve wasted too much time on a fool’s errand, tracking them down when they should have been rebuilding their society. After all this time, the only thing the Warriors understand is what they can win by conquest. Just as the Doctor thinks he might be winning Isskar round, they’re interrupted by the arrival of the Valdigian guards who’ve come to rescue them. The Doctor and Amy dive to the ground to avoid being caught in the crossfire as soldiers from both sides open fire.

As the battle grows in intensity, a fully recovered Lady Mesca offers military advice to the Valdigian army. Conscious of what the Doctor told her earlier, she arranges for hot fins to be fired at the Ice Warriors using the springs on their scaffolding. Eventually they’re able to retrieve the Doctor and Amy uninjured, but the Doctor warns that they won’t defeat the Warriors this way, only slow them down. Mesca refuses to let the invaders take the castle and sends Thetris to pin the Warriors near the doorway and trap them there. Unfortunately Isskar has ordered support from his heavy artillery troops. He gives them permission to kill everyone, except for the Doctor and Amy.

Behind the front line there are many wounded Valdigians, among them Wembik. She insists that the Doctor help the others first, but he examines the damage to one of her wings and realises how badly hurt she is. She tells him that if the membrane is exposed, her wing will have to be removed. The Doctor hesitates, knowing how much pain it will cause, so she rips it off herself. Amy reports that the Ice Warriors have got bigger guns and are starting to push forward. She says they can’t possibly look for the second segment while there’s a war going on, but the Doctor’s priority is to help the wounded people move to a safer location. Amy helps Wembik to her feet and leads her and the other wounded insects towards the TARDIS.

The Doctor tells Mesca she has to call her soldiers back as there’s no hope of forcing the Ice Warriors from the castle. Mesca refuses to let her people be conquered, but the Doctor says it’ll only lead to more of them getting killed. Wembik joins them and asks for permission to continue fighting. Mesca agrees, then she leads her soldiers in a reckless charge against the Ice Warriors. Incredibly, their show of force is a success and Isskar is forced to withdraw his men. The Valdigians celebrate, but Mesca immediately sets them to work building barricades and rounding up any prisoners. The Doctor is amazed and tells Mesca how brilliant her leadership was. She agrees, but points out that the advice was actually coming from Wembik. The Doctor is pleased to see this has helped to bring the warring factions together again and Amy thinks it might be a good idea if aliens attacked more often.

The Doctor says they need to find the second segment and it’ll probably be in the form of an artefact that signifies strength and endurance. Unfortunately the Valdigians have nothing like that here - their forebears arrived with nothing more than the brood on their backs. The Doctor knows it has to be something in the castle and when Amy says she’ll recognise the segment when she sees it, Wembik reveals that Zara told her exactly the same thing when she warned her about the Doctor and Amy. Mesca announces that Zara is the spirit of the castle and she’s been visiting them for years and has still never found what she seeks.

The Doctor asks if he can speak to a prisoner and is shown to the dungeon where Amy was kept earlier. He learns from one of the Ice Warriors that Zara has been feeding Lord Isskar with useful information for a very long time in return for his help in capturing him and Amy. He realises Zara has been unable to find the segment too and she needs to make sure he and Amy don’t get to it first. When he returns to the others, he discovers Amy has resolved the differences between Mesca and Wembik by encouraging them to marry each other. That way it doesn’t matter which boy becomes King as they’ll both be one of its parents and be able to share in the honours. Thetris is momentarily speechless, but then says he understands and gives his support. However, they still plan to go to war - not with each other, but united against the Ice Warriors.

The Doctor approaches Lord Isskar and they face each other across the drawbridge. The Doctor invites Isskar and three of his Warriors back inside the castle and once they get across the threshold, Isskar asks the Valdigians to lower their weapons as a gesture of peace. As soon as they do so, the Ice Warriors grab Amy and take her back outside to be held as a hostage for Isskar’s safety. The Doctor tells the Ice Lord that Zara has been tricking him. Isskar was warned that the Doctor would try this tactic, but he reveals that he doesn’t trust Zara anyway. The Doctor explains that because of her, the Valdigians nearly went to war over a trivial ceremonial role, but she’s also been popping up every few years and making suggestions to Isskar which have done nothing but appeal to his worst side and stir up his frustrations and fears. The Doctor and Mesca have both deduced that it was Zara who suggested the snake venom to Wembik - and all because Zara hasn’t been able to find what she was looking for. The Doctor reveals that he now knows where the hidden segment is…and in that instance, Zara appears beside him, exactly as he’d guessed she would.

From across the drawbridge, Amy insists on being allowed to rejoin her sister so she can hear what she has to say. Now that she’s outside the castle, she’s noticed that the overwhelming tickling sensation has completely disappeared. She now knows why she was getting confusing signals and as her head starts to clear, she can finally see the segment…

Mesca orders her soldiers to open fire on Zara, but it doesn’t do any good as she’s protected by the segment she’s carrying. The Doctor has noted that Zara and Amy are both easily influenced - they watch other people and work out how they behave - but Zara has learned how to make other people copy her. He taunts Zara and accuses her of being really stupid, claiming he’s already worked out where the segment is and he’s only been here half an hour. Now that Zara’s influence over everyone has been revealed, it no longer works and she’s unable to get Isskar to do her bidding.

Outside the castle, the Ice Warriors reveal that their orders are to kill Amy if they’ve not heard back from Lord Isskar within two minutes. Ignoring Amy’s protests, the Warriors activate their sonic blasters and attempt to storm the drawbridge. The Valdigians fall back behind the barricade and allow the Warriors to enter without returning fire, so the only person hit by the sonic vibrations is Zara, but she laughs it off. She prepares to leave and when the Doctor tries to stop her, Isskar places him in custody. Zara strides towards the Warriors and they fall back, powerless and unable to harm her. She’s soon reunited with Amy outside and she too notices how clear her head has become now that she’s outside the walls of the castle. When she turns back round she sees the segment and realises it was the castle itself. The reason this location was so effective for peace talks is that it made the people inside want to achieve balance.

The Doctor fights the pain and calls out to his fellow prisoners, urging them to concentrate on anything that will help keep their minds focused. He recites to himself the story of the three little pigs, but all the facts seem to get confused inside his mind. He knows he won’t be able to keep this up forever and he pleads with Amy to act fast…

The remaining Ice Warriors are shocked to discover Zara has destroyed the entire castle with just a single touch of her hand. They open fire on her at point blank range, but again their sonic blasters have no effect, so they return to their ship to get new orders. Amy asks Zara to return the castle to its former state, but she’s not sure she can. Even if she could, she says the segment was decaying and it would have been dangerous to leave it where it was. She says she can actually hear all the people inside the crystal. Satisfied with her success, she decides to get the remaining segments too - including the one inside Amy’s satchel. Although it’s protected inside its own dimension, Zara is confident she can use the power of the other segments to retrieve it, but Amy reminds her that they’re both supposed to balance each other out. Suddenly the power that Zara was summoning blasts back at her and she’s surprised to find herself bruised and bleeding. Even worse, her satchel has broken and she now has to carry all the segments. Amy finds that one of the segments has appeared in her hand and she believes it was attracted to her as a way of balancing things out. Now they have two segments each and nothing Zara says will convince Amy to hand hers over. Furious, Zara attempts to reach inside Amy’s head and make her do her bidding…

Zara can sense that Amy liked the Doctor and not just in a platonic way. Amy denies this, but Zara can see a moment when the two of them held hands while they were fleeing from the Ice Warriors. However, the process works both ways and Amy sees Zara and Zinc flirting with each other. Zara is surprised when she sees what she looks like for the first time and realises that in Amy‘s eyes she‘s become a monster. She claims that she chooses to be that way as a deliberate counterbalance to what the Doctor has taught Amy.

The Ice Warriors return and order Zara to attend a tribunal, but she still has Zinc’s time bracelet and uses it to disappear. Amy shows the Warriors the crystal in her hand and tells them it’s the one containing the castle and everyone who was inside it. She tries to call to the Doctor, but his mind is so consumed with the stories he’s concentrating on that he can’t respond. Eventually he hears her voice as a faint echo and tells her she needs to hurry as there isn’t much of him left.

Amy tells the Ice Warriors that she’s going to release everyone, including Isskar, but they remember Zara warning them that the castle was dangerous. If she releases the power of the decaying segment, she could kill all of them. When she refuses to sacrifice the people trapped inside, the Warriors agree to fall back to a safe distance. Amy concentrates and slowly the light becomes an enormous flash…and the castle is restored and back to normal.

The Ice Warriors summon a medic to attend to Lord Isskar. Amy races over to the Doctor and tries to bring him round, but he’s babbling incomprehensible nonsense. After letting him rest for a while, Amy brings the Doctor a cup of tea and tells him the TARDIS is also fine and is being brought up from the corridor where it landed. The Doctor believes that having the TARDIS inside as well is what helped him keep things together and why he‘s not in as bad a state as the others. He said it was like being aware, yet not having anything to latch onto. He now realises what it must have been like for the people that the segments were disguised as.

Amy checks with Wembik who assures her everyone has been accounted for, then she reaches out and transforms the castle back into the crystal segment. The Valdigians are disappointed that the Doctor is planning to leave already as they were planning to hold a banquet to celebrate Mesca and Wembik’s wedding. The Doctor says they might pop back later, but there are still two more segments for them to collect - but as he and Amy turn to leave, Isskar stands in their way and says he’s had the TARDIS loaded aboard his ship. He still has orders to put them before a tribunal where they must explain their actions. If they won‘t obey him, he’ll order his Warriors to open fire on the Valdigians. The insect people say that they’ll fight the Ice Warriors if they have to, but the Doctor knows they’d be wiped out in an instant so reluctantly he agrees to go with Isskar. Mesca believes Isskar is going to kill them anyway, but he says he has no reason to do that and in fact he hopes they’ll have the peace that's been denied to his own people.

The Martian spaceship lifts off from Safeplace. The Doctor says they’re travelling faster than light and back on Safeplace, Mesca and Wembik’s great grandchildren will have already reached old age. The Ice Warriors insist they hand over their belongings, and although Amy refuses to give up her satchel willingly, it‘s taken from her by force. They’re locked up in a cell but the Doctor asks Amy if she can reach out and make contact with the power inside the segments. After seeing what it did to Zara, she’s worried she might not want to stop and will become corrupted, but the Doctor says he has faith in her and all he wants her to do is open the door. He notices she’s blushing, which is something else she couldn’t do when they first met.

Amy manages to open the door and is excited at the prospect of how much power she now has. She follows her senses and leads the Doctor to where the segments are being kept on an upper floor - but they‘re immediately caught by one of the Ice Warriors. They’re escorted to the bridge where they find Isskar had deliberately locked them up so Amy would be forced to demonstrate her power. He reveals that he has another guest - Zara - who‘s promised him she will use her segment to restore Mars to the way it was. Zara wants Amy to join up with her, and when Amy refuses to leave the Doctor, Zara offers to make the decision easier by destroying the ship and everyone in it. Isskar realises he’s been betrayed, but Amy can see inside his head and points out to him that he’s wasted his life in the search for revenge. If he’d loved his people so much, he should have stayed with them and helped them rebuild.

When Amy refuses to join Zara and says she’d rather stay behind and die, Zara prepares to leave with the segments…but then she realises she can’t sense them any more. Isskar reveals that after he let Amy test her power in the cell, he had the segments placed within a strong box whose interior is outside time and space. Zara says she will destroy the ship and scour the wreckage for them. She uses her power to disable the spaceship controls, setting them to overload, and then disappears.

The Doctor desperately tries to repair the damage Zara has done, but none of the controls are working. He suggests evacuating everyone to his TARDIS, but Isskar reveals that Zara ordered him to dump the TARDIS while they were in hyperspace to prevent him escaping. At the Doctor’s instructions, Isskar orders his crew to abandon ship, but the Doctor decides to stay behind to try to shut the warp engines down. The ship is rocked by an explosion and Amy reports that there’s a breach in the lower hull. Isskar refuses to leave without the Doctor and Amy, but they want to retrieve the segments first. Isskar produces the strong box which was hidden under his chair. The lock is empathic and only opens to those with pure thoughts, so Amy is delighted when it immediately opens at her request.

As they leave the bridge they encounter an Ice Warrior who reports that several of the escape pods have been damaged by fire and are unusable. They search around until they find one that looks functional, but there’s only enough room for three. Isskar refuses to leave, but the Doctor tells him it’s important he return to his people and tell them what it used to be like to be a Martian. Isskar still refuses and physically tries to manhandle the Doctor into the escape pod in his place - but then the other Ice Warrior grabs hold of his elderly leader and drags him into the pod against his will. Amy joins the Doctor, knowing that the segments will protect her, and together they seal the door to the escape pod and eject it into space. Amy tries to use the segments to repair the ship, but she doesn’t know what to do. The Doctor notes that the problem with power is that it’s always easy to break things, but mending them takes skill. The ship is about to burn up and there’s nothing they can do to stop it, so the Doctor suggests they return to the bridge as at the view will be better from there.

Resigned to their fate, the Doctor and Amy watch the beautiful spectacle as the ship heads at twice the speed of light into the Laboon Constellation. It used to consist of 47 planets, but it’s only 46 now after there was a war with the Cybermen. The Doctor tells Amy he’s ready to die. They know she will be protected by the segments, but there seems to be nothing they can do to save him. The shields on the spaceship give way and the temperature starts to rise. Suddenly the Doctor has an idea and races over to the controls and starts pressing buttons - but before he can complete his task, everything goes quiet. Amy congratulates him, but he says it was nothing to do with him. So who saved them?