Vesta rises before the Guide's Bell and dresses for the cold. She puts on layers of warm clothes for the winters have been growing more severe. She had never seen snow until three years ago but now it lies thickly every winter and gets worse every year. She thinks about the story of things lurking in the woods but dismisses the idea as she attaches a lamp to her pole. She knows that by going to put flowers on her father's grave she will miss an hour of work but she also knows that the others in the village will be confident that she will make the time up later. She leaves Beside and follows the North Lane out to Would Be. The trees are silent but she has a feeling that something is following her. She presses on across the snowy landscape until she reaches the Memory Yard in the middle of the wood. She lays her flowers at her father's grave just as the Guide Bell rings out in the village below. She asks the Guide to look after her dad and then sets out for home. She sees a star move across the sky. There has been much talk recently of moving stars, so she hurries on down the path to see if she can catch another sight of it. The villagers of Beside think the stars and the winters are bad omens but Vesta isn't superstitious. She stops short when she sees the tracks in the snow, deep footprints with an enormous stride. A giant has passed by through the woods very recently. Suddenly scared, she finds the idea of her father's grave reassuring and turns back the way she came. Something growls in the trees.

Amy compliments the Doctor on a perfect landing, albeit that the TARDIS seems to be leaning slightly. He has promised Amy and Rory a trip home for Christmas but as they emerge into a silent and snowy rural scene it is apparent that they are not in Leadworth. The Doctor says that, wherever they are, it is definitely Christmas. He strides off with the other two struggling through the snow behind. There are three mountains nearby, although the Doctor says that he doesn't think they are mountains really. He doesn't elucidate further but carries on down the slope. Rory ask the other two to wait while he runs back to the TARDIS for a coat. As soon as he has gone, the Doctor and Amy are confronted by six men, grim faced and carrying pitchforks.

Rory returns to the same spot to find his friends have gone. He notices that another set of tracks have joined theirs and everybody seems to have gone off together. With a sinking feeling, he decides to follow. Soon, he is horrified to see four or five green giants walking towards him. Knowing that he must avoid them, he runs.

The arrival of strangers in Beside is treated with some trepidation. The Elect, Bill Groan, hopes that they are well-wishers from one of the other plantnations but neither the Doctor nor Amy is dressed like Morphans. His adviser, Winnower Cropper, the last of her generation, remarks that they have not received well-wishers at festival time since the ice started coming. Bel Flurrish wants to speak with them regarding the disappearance of her sister that morning but Bill says he wants to question them first. The Doctor and Amy are led to the Assembly Hall. The Doctor is intrigued by the building and its construction, apparently of something called 'shipskin'. Bill Groan introduces himself as the Nurse Elect. The Doctor gives his name and Amy's but cannot explain where they come from satisfactorily. He shows his psychic paper to Jack Duggat who reads that they have come as festival well-wishers from Seeside Plantnation. Far from smoothing things over, it makes everything worse: Jack Duggat cannot read.

The Doctor and Amy are locked in a cell beneath the hall. Bel demands that Bill Groan question them about her missing sister but he says there must be a council meeting first. Bel comes down to them, holding the psychic paper. She demands to know why it contains a picture of her sister. The Doctor says that one of its drawbacks is that it shows people what they want to see. He and Amy ask how they can help her. She tells them that her sister left in the night and has not been seen since. The Doctor seems more interested in the sudden onset of harsh winters but he does tell her that if they are released from the cell they will help her look for Vesta. She replies that she doesn't have the key but he opens the lock with his sonic screwdriver anyway.

They sneak out of the building and back to Bel's house. He is again intrigued by the wooden walls and shipskin nails. He finds that the Morphans have lived on Hereafter for twenty seven generations. It seems, he says, as though the terraforming programme is failing somehow. Bel tells him that other things are going wrong, too: livestock going missing, people in the woods who cannot be accounted for, stars that move across the sky. She suddenly remembers that this is the anniversary of her father's death and knows where Vesta must have gone.

Rory runs down the snowy slope into the trees. He suddenly finds a group of men around him. They are carrying axes and pitchforks. He tries to tell them about the green giants but all they care about is the whereabouts of Vesta Flurrish. They are about to drag him back to the council when one of the green men steps into the clearing. One of the men strikes it with his axe but the blow has little effect. The giant throws the man back against a tree with a bone crunching impact. The other men leap in to attack the creature and it uses a weapon on its forearm, some sort of sonic gun. One man is killed instantly and the others turn to run. As Rory flees he sees another man near him shot down.

Out on the North lane the Doctor pauses. He seems to have heard something. Bel says that it is the dogs barking in the village below but he says that he has heard something familiar but forgotten. Before he can think further they are accosted by Samewell Crook.

Rory hides behind a tree. As far as he can tell, all of the other men have been killed. The green monster draws near him and looks around. Somehow it seems to see him. He wonders if it is a reptile and then concludes it has acute sight or heat vision. He runs but it doesn't fire at him. As he hears its remorseless footsteps behind he wonders if it wants him alive.

Samewell tells Bel that freeing the prisoners is a Cat A crime. She disregards him; finding her sister is paramount. It is obvious to Amy that the young man is besotted with Bel. The four of them arrive at Would Be (or Wood B as the Doctor assumes it was called originally. He finds that the three plantnations on Hereafter are Aside, Beside and Seeside. He says that this is an earth colony, late Diaspora, and speculates that the Earth has already gone.) the Doctor is worried that the winters mean that the world is becoming less Earth-like in a way that he has seen before.

Rory races down a slope and sees a river of ice in front of him. He jumps onto it, hoping it will take his weight but not that of his pursuer. Half way across the ice gives way and he plunges into the water. To his surprise, it is warm. He is carried along beneath the ice and realises that he will be suffocated all too soon. To his surprise, the ice ends and he floats into a large mill pond. The current carries him to some buildings where he scrambles onto a jetty and looks for a door into one of the buildings. He hears a sound on the jetty and puzzles how his pursuer could have got there so fast. He finds a hatch and crawls through, locking it behind him. No sooner has he done this than something scrapes at the handle on the outside. He hears the green giant's weapon being fired, then a thump, then silence. After a long pause and no further sounds he crawls deeper into the darkness. Just as he realises that there is somebody in the dark with him he is knocked unconscious by a blunt object.

Thick snow is falling by the time they reach the graveyard. The fresh flowers on her father's grave tell Bel that Vesta was here. Vesta's are the only tracks but they are disappearing under the snow fall. The Doctor follows them until he gets to the giant footprints that cross Vesta's. The Doctor follows the new trail and finds a clearing where the snow is heavily and widely stained with blood. The Doctor says that whoever shed the blood is obviously dead but Samewell discovers sheep's vertebrae and says it is another of their animals that has been killed. He blames a hungry dog but the Doctor is more suspicious: a dog could not have consumed a whole sheep in so short a time. The snow is now so heavy that they decide to find shelter. Samewell leads them in the direction of a vent he knows is nearby. The Doctor assumes this is some sort of protection against the wind.

Rory wakes up to find he has been attacked by a girl holding a wooden mallet. She says she came into the building to hide from a creature with red eyes. Rory assumes that this was the same, or similar , creature that he was pursued by. They introduce themselves and Rory finds that she is Vesta Flurrish. He tells her that he met some men who were looking for her.

The snow eases as night falls and clouds begin to break up. Bel shows them a patch of stars. One of the stars is moving east to west. The Doctor tells Amy it looks like a spaceship in orbit. Amy asks if the planet is being invaded but he says that the invasion began months earlier - the people are only just beginning to notice it. Six figures close in on them from all sides. They are green and massive. The Doctor says that his hunch was right and identifies the creatures as Ice Warriors.

The Ice warriors fire a warning shot and tell the four people to surrender. Instead, the Doctor sets his screwdriver to block the sonic guns and the four of them run off through the snow. Samewell leads them to a conical structure the size of a hut. It is made of metal but, once inside, the Doctor does not hold out much hope of it keeping out the Ice Warriors. He says that they are honourable creatures but dangerous enemies. As the Warriors begin to bash at the door of the hut, the Doctor looks for and finds a trap door. He says that they are at the top of a ventilation shaft. They drop down and slide down a big, shiny tube for what seems to be miles. Amy, the last to go, gets separated from the others. When she arrives at an intersection and stops, she has to make a choice which way to go. She sets off walking through another tube and is confronted by a pack of rats. They are large, naked and have too many legs, but they are still rats to her. And they look very hungry.

Vesta and Rory swap stories of how both of them separately ran away from red-eyed monsters and ended up hiding in what Vesta calls the 'autumn mills'. She says the water that Rory fell into was warm because it runs out of Firmer Number Two. She guesses that Rory is a shepherd but when he says that he is a nurse she is embarrassed, assuming that he means a Nurse Elect (or village leader). When she assumes that he came from another plantnation for the Winter Festival he does not disabuse her. She insists that they go to Beside at once.

Amy runs from the ravenous horde of rats. Just as they seem likely to overcome and eat her, she is deafened buy a piercing noise that kills some of the rats and drives the others away. She sees the Doctor approach, brandishing his sonic screwdriver.

Sol Farrow is on night-time sentry duty at Beside's west gate. He reflects on the fact that there are now sentries posted all around the village, not least because of the disappearance of so many people that day but also because of the lost livestock and the rumours of giants in the woods. In the assembly hall a meeting of the villagers has been called to discuss the disappearance of eight men, the Flurrish sisters and the two strangers. Winnower tells Bill Groan that he must face the people but he is at a loss for what to say. She suggests that they consult the Guide for advice. Before they can go in, Sol arrives with Vesta and Rory.

The Doctor tells Amy that the rats were purpose built by the terraformer when they arrived. The three mountains that the travellers saw when they arrived are atmospheric processors full of extraordinary machinery. He thinks the rats would have been brought or (more likely) the technology for their making would have been brought from Earth. When the machinery detected intruders within the pipes, or a loss of efficiency, it would have made and released the rats to clear out the glitches. He thinks that it might have been escaped rats that have eaten the livestock. He thinks that the lack of efficiency is actually the Ice Warriors tampering with the systems for their own benefits.

Vesta introduces Rory to Bill Groan and the assembly as her new friend. She tells them that the Doctor and Amy are Rory's friends and that he is a well-wisher from a plantnation they have never heard of called Leadworth. Rory tries to cut across this side-tracking to talk about the creatures with the red eyes. He says that one of these creatures killed men from the village. Pandemonium breaks out in the assembly as villagers try to learn of their loved ones. When Vesta tells the crowd to respect a Nurse from another village Bill and Winnower lead Rory to another room: the antechamber of Guide's room, the Incrypt. They want Rory to prove himself by opening the door to the Incrypt. To do so he presses his hand against a palm reader and the door opens. Having proved himself, Rory is led back to the assembly hall with Vesta. The room has been cleared now. Bill and the council members enter the Incrypt while Vesta and Rory stay in the assembly to eat soup.

The Doctor leads his friends through the massive chambers and tunnels of the terraformer. He says he needs to find out what the Ice Warriors have been doing and to stop it. On their journey they find a room full of decaying organic matter. Something has cut through the doors to enter a mile-long laboratory full of incubation tanks. The tanks have been sabotaged and the meat that was being grown has gone off. They leave the room but very soon spot a group of Ice Warriors approaching. There are three of them and they have replaced their sonic blasters with huge broadswords. More Ice Warriors emerge from other tunnels. The Doctor races down a long corridor to a door but finds he can't pass through because the palm reader does not recognise him as human. Bel opens the door for him and Amy and Samewell pass through. The Doctor stops to address the Ice Warriors. He tells them that he is the 'Belot'ssar' and he greets their warlord. This stops the pursuers briefly and allows him to pass through the door and Amy shuts it. They keep on running and arrive at a work station.

The Doctor sits at some controls and brings the equipment back to life. With a flourish, he presses a button and suddenly the world around them changes. Rory, meanwhile, is in the assembly hall with Sol. He comments that the council are taking a long time over their discussions. Sol says that is to be expected when the Guide Emanual is being consulted over something unexpected. Just then, the room changes to resemble a modern white chamber. Rory finds himself facing the Doctor and Amy, as well as two Morphans he has never seen before. The Doctor tells him that he isn't really there: it is a virtual projection. Vesta and Sol are terrified of the apparition. Rory says that the village is scared by the appearance of monsters with red eyes. The Doctor says that they are called Ice Warriors and they are a threat to all human life on the planet. Their conversation is repeatedly interrupted by the sounds of the Ice warriors trying to get through the doors nearby. The Doctor presses on, saying he needs to recalibrate the Terra Morphers back to their original settings. He says he needs a copy of the Guide. Rory adds that the council are consulting an E-Manual as they speak.

Ice Warriors burst into the chamber. Amy is dragged away by Bel and Samewell but the Doctor is grabbed by one of the Warriors as he tries to disable the terminal. He is pinned into his seat by an axe-wielding Ice Warrior who tells him that death is inevitable but there must be talk first. An Ice Lord enters the chamber and demands to know what weapons the Doctor has. When the Doctor tells him he has no weapons, the Ice Lord does not believe him, saying that new weapons have been deployed against the Ice Warriors that day. The Doctor admits that he used his sonic screwdriver to overcome the effects of the sonic disruptors and shows him the screwdriver, now drained of power. The Ice Lord mentions other occasions when his men's weapons were rendered useless but the Doctor has no idea what he is talking about, though he realises that this is why the Ice Warriors have reverted to swords and axes.

The Ice Lord says that they found the planet ten years earlier and found it ideal for settling. After observing the colonists, they decided to change the climate. However, recently they have seen new arrivals and sent troops to identify them. The Ice Lord says that recent events have turned the matter into open war. The Doctor thinks that the Ice Warriors have misread the situation. During their interrogation it becomes apparent that the Doctor has had previous meetings with the Ice warriors and knows of their bloodlines and clans. The Ice Lord wonders why the Doctor refers to himself as 'Belot'ssar', meaning 'cold blue star'. The Doctor says that it is name given by the Ice warriors to the TARDIS and is a term showing his friendship with the Ixon Mons family, given to him by Ice Lord Azylax as a mark of respect. Unfortunately, this Ice Lord, Ixyldir, has never heard of any of this, including Azylax. The Doctor realises, with horror, that he has arrived a lot earlier in the Ice Warriors' timeline than he thought by about 9,000 years.

Amy tells Bel and Samewell that the Doctor allowed himself to be caught so that the others could escape. This means he must have had a plan. She knows that this must involve sabotage and getting the Guide. Samewell points out that the most pressing thing to do is evade the Ice warriors that are closing in on them.

The assembly hall reverted to itself just before the Ice warriors arrived. Rory is in a panic, wondering what has happened to Amy. The council emerge from the Incrypt, asking what has happened. Rory tells him that the planet is under attack and that the Doctor needs the Guide to save them all. Winnower refuses but before a decision can be made, a man bursts in and tells them to come outside. They step out to see three stars moving across the sky and a fourth descending towards them at speed. Rory tells them that it is a space ship. At the same moment, the sounds of battle are heard nearby. The villagers have no idea who can be fighting. Energy weapons are fired from the space ship at targets in the woods. Rory tells Vesta that this must be the Ice warriors. She wonders why they don't use their teeth and talons instead, at which point it becomes clear to them that they have been talking about two completely different sets of red-eyed monsters all along.

The Doctor tries to persuade the Ice Lord that his people have a strict code of honour and that, by stealing a planet, they are contravening their beliefs. He argues that the colonists have no way of defending themselves but Ixyldir contradicts him, saying that one hundred and fifty new colonists have appeared, each capable of resisting the Ice Warriors.

Amy and her friends have to leap from one walkway to another to escape the Ice Warriors. They then find themselves face to face with a new creature. Back in the village of Beside, two Ice Warriors are seen fighting a lithe, loping humanoid creature.

The Doctor persuades Ixyldir that they need to find another control room with better access to the systems in order to get the situation under control. The Ice warriors say that there is such a place on level six. As they make their way there, the Ice Lord explains how his people need a home and began to tamper with the Terra Formers. This initially brought a response in the form of vermin but the resistance has escalated. The Doctor says that he suspects the defence mechanisms created a new weapon: Transhumans. This is what the Ice Warriors are fighting now.

The new creature is part human, part quadruped and a lot of machinery. It has steel teeth and claws. It bypasses the humans and fights with two advancing Ice Warriors, killing one. The other Ice Warrior, mortally wounded, holds onto the Transhumant and leaps off the walkway, taking both to their deaths.

In Beside, the new Transhumant approaches Winnower and tells her that it has come to protect the Guide System. It follows them to the assembly hall. Winnower says that she didn't mean for this to happen: she only asked for help from the Guide. Before Rory can ask her what she is talking about, a hologram of the Doctor reappears. He tells Rory that the Transhumans don't care about the Morphans; their job is to protect the future of the planet for the real colonists who are in hiber5nation in the Firmer (one of the three mountains). They are the elite from Earth who are using the Morphans as their labour force until the planet is ready for them. He says it is the Transhumans that have been killing and eating the livestock. The Transhumant agrees, adding that once all the livestock is eaten, the colonists will be next on the menu. The Doctor looks up from his console and sees four Transhumans approaching, bringing Amy, Bel and Samewell as their prisoners.

The leading Transhumant tells the Doctor to cease his interference but adds that this won't save anyone's lives. However, the Ice Warriors have laid a trap and spring from hiding places. A vicious fight ensues. The Doctor shouts at Rory to act now. Rory runs into the Incrypt and orders it to send data to the Doctor's location. As the battle carries on around him the Doctor hammers in new commands and presses 'activate'. Meanwhile, Rory and Jack Duggat are fighting off their own Transhuman. Suddenly, it retracts its claws and turns to depart. Those fighting the Ice Warriors do the same. The Doctor says he has sent them back to hibernation.

The Doctor tells Ixyldir that there is a planet, Atrox 881, that he knows will be a significant part of the Ixon Mons dynasty in 9,000 years. It is circling a cold, blue star.

Bill Groan asks if the travellers would like to stay. The Doctor thanks them but says he must be on his way. He warns them that it will be a few years before the hard winters cease but he doesn't expect the Morphans to encounter too many problems. He says that he has reset the Guide controls so that the Morphans can all access the data base easily. He adds that he has reset the hibernation equipment so that the sleeping colonists will not be released until - or if - the Morphans decide.