Late March, 1963. Susan and her grandfather have been in London for months, and in an attempt to fit in, Susan has chosen to attend school. This has proven to be a mistake. The teachers are pedantic, her form master is authoritarian, and the lessons are childish to someone of her background. Her only friends are Gillian Roberts, whom Susan suspects is abused at home, and John “the Martian” Brent, a bespectacled boy whose father is a captain in the British army. A school bully named Francis Minto makes life miserable for these misfits; one day he swats the head off the snowman which Gillian had made, but the damage is easily repaired. Susan doesn’t feel as though she’s fitting in at all, and her grandfather offers little sympathy. She isn’t sure why, for she feels certain their departure from home was an act of rebellion -- and yet she can remember little about her time there.

If indeed it was “time” which passed. Somehow she feels that Time works differently at Home, and that time only really began to pass for her and her grandfather when they stepped out into the Universe at large. Whenever she tries to think about Home, however, she gets terrible headaches, as if the knowledge has been blocked from her mind. The only person she can open up to is Malcolm, a little boy whose parents come from Trinidad; Susan looks after him on weekends while his mother works at the newsagent’s stand. Somehow, it’s as though telling the stories to children doesn’t count, and Susan also finds eventually that she can think about Home by dressing it up in the language of the present. It feels as though she and her grandfather have skipped school and are being pursued by the Truant Officers, particularly one Master who was always the teachers’ favourite -- a dark, bearded man whose career will be made if he brings them in. But even as they try to rebel, some rules remain hard-wired into their very beings, and the most important one is this: they must never interfere.

As if this wasn’t enough to worry about, the world is experiencing an unseasonable cold spell which shows no sign of letting up. There’s fresh snow and ice on the ground every morning, and the army is called out to deal with the emergency as it becomes evident that this cold can’t be natural. The Americans blame the Russians, claiming that this is the result of the top-secret Novosibirsk Project; the Russians claim that the Project is merely intended to help grow crops in Siberia, and that American oil drilling in Alaska is to blame. In short, nobody has any idea what’s happening, and people are starting to become frightened. Susan’s grandfather has also noticed the cold, but he’s treating it as an abstract problem, not something which could affect him personally. This disturbs Susan, for it’s just how she thinks the people at Home would behave -- and if her grandfather is really like them, then why did he run away?

Despite Susan’s efforts to fit in at school, even Gillian and John are starting to notice that their friend is odder than they’d thought. Susan can’t quite put her feelings into words, and gets very angry when a soldier on the street calls Malcolm a “golliwog” -- and his sergeant refuses to upbraid him for it. She tries going out on the town with Gillian on Saturday night, and the two girls draw a biting look from the vicar’s wife when they emerge from the Youth Club washrooms, bright with eyeshadow and lipstick. Susan doesn’t feel comfortable in makeup, but she accompanies Gillian anyway, to a pub called the Pump just outside Totter’s Lane. There, Gillian flirts with a motorcyclist named Zack, who loses interest when Susan inadvertently admits that they’re underage. Gillian is furious and storms off, leaving Susan to return home alone... but on her way back to the junkyard, Susan senses an intelligence watching her from the ice. The sensation seems stronger around the Box in which she lives, and Susan begins to fear that the unnatural cold may have been caused by her grandfather’s tampering with the Box’s controls. But the next day when she emerges from her room, she sees her grandfather standing in the yard, speaking to a wall of shifting, moving ice -- and apparently promising not to get in its way any further as it rids its world of the pestilent human beings.

Monday, 1 April, 1963. Gillian seems to have forgiven Susan for Saturday night, but Susan is more concerned with the Cold. During break, Francis Minto tries to smash the snowman again -- and it comes to life and stabs him through the leg with claws of ice. It falls dormant before it can do anything more, and the terrified Francis accuses Susan and Gillian of stabbing him. They are given detention and notes to their parents, and Gillian is furious when Susan tries to protest on her behalf, knowing what her father will do to her; it’s as though the humiliation of having it spoken out loud is worse than the punishment will be. After school, Susan and Gillian are sent up ladders to melt the ice away from the gutters while the boys work below, supervised by Mr Okehurst -- but as the sun sets and the temperature drops, the snow comes to life and attacks them all. Gillian and Susan save themselves by breaking through a window into the school, but Okehurst and the boys are slashed to death by spears of ice.

John is inside, as ROTC training was cancelled due to the cold but his father, unaware of this, has not come to collect him yet. The three friends watch as clumps of snow and ice rise up beside their snowman to create soldiers -- “Cold Knights”. When Captain Brent arrives to fetch John, the Knights attack his car, crushing his driver to death. Brent holds them off with a blowtorch until John, Gillian and Susan can get him into the school, but the shock has sent him into denial. They shelter in the boiler room until morning, and emerge to find that only a few children and teachers have shown up; rumours are flying about strange events during the night. The snowman is still standing in the yard, and John tries to shoot it with his father’s gun, but the bullet has no extraordinary effect. Susan realises that the snow only comes to life when it’s cold enough -- and the temperature could drop again at any time. The adults can’t bring themselves to believe this story, but the children take matters into their own hands, and form into groups to make their own ways back home.

Susan manages to convince Gillian and John that her grandfather can help them, and they set off for Foreman’s Yard, along with Captain Brent, who is still in shock. But the wind picks up and snow starts to fall, and the temperature begins to drop. In the High Street, they encounter a patrol of soldiers who claim that last night, people in Piccadilly Circus were attacked by the snow -- and then three snowballs, the remains of the school snowman, roll down the street from Coal Hill, pursued by an avalanche which kills all of the soldiers. Cold Knights form out of the snowballs and surround Brent and Gillian, and to John’s horror, his father gives his life to fling Gillian out of the way before the Knights attack. Susan and Gillian manage to get John moving again, but the storm becomes worse, and even the small snowflakes begin to sting their exposed skin. They seek shelter in a Wimpy hamburger bar, where they are reunited with Malcolm and Zack. The Haighs, the vicar and his wife from the Youth Club, are also sheltering here, but they seem to have snapped, and Mrs Haigh accuses Malcolm and the girls of bringing this judgement upon them with their loose morals and heathen ways. The windows begin to crack from the extreme cold, and Mrs Haigh tries to grab a knife and sacrifice Malcolm to her angry God. The waitress Dolly gets Malcolm away from her before she can do more than tear open his Cowboy Gonk toy, and Zack comforts him by speaking with a western drawl and pretending that the Gonk is still “alive”. Together, they set off into the snow, leaving the Haighs to fend for themselves.

Susan and her friends make it to the Railway Bridge, where they see bodies of people frozen in the ice. They manage to cross the rails without incident, but just as they think they’re safe on the other side, Cold Knights burst out of the station and crush Dolly to death. The others escape, only to find Totter’s Lane blocked by a wall of snow which grows teeth of ice when Zack tries to climb over it. They seek shelter in the pump, only to find that the cold has frozen people into their seats and burst open the liquor bottles. As Zack tries to break open the back door, John finally lashes out at Susan for his father’s death; he’s seen enough to realise that she’s different from them, so what does she know about this? She is forced to admit that she’s from another world, but while she insists that she has no connection to the Cold, she knows that her friends will never look at her the same way again.

Zack finds the back door blocked by a wall of snow, but it parts for Susan, allowing her and her friends into Foreman’s Yard. It appears that her grandfather has made a deal with it, but that’s not an entirely good thing -- for the device which he’s connected to the Box is making things easier for the Cold. He explains what he’s doing to Susan, but doesn’t seem to notice the humans with her unless they address him directly. It seems that the Cold is an animus of intelligence which inhabits frozen water; it lived on Earth during the last great Ice Age, but sank into dormancy as the climate changed. It has been summoned back to life, perhaps by the Novosibirsk Project, perhaps by the American drilling, or perhaps by both -- or perhaps just because it was its time again. Now it plans to cleanse the world of the infestation of human beings, but the old man’s meddling with the Box’s controls was inadvertently drawing energy away from the Cold as it tried to re-establish itself. However, he has now built this machine to correct matters. If he chose, he could use it to draw the animus of the Cold into a single snowflake and remove it from this world -- but that would be interfering, and that is something he cannot do. Soon the world will belong to the Cold again.

Susan protests, knowing that this is not truly part of her grandfather’s character; he’s just obeying a rule which has been impressed upon him by others, so deeply that he can’t consider breaking it. Gillian threatens to kill Susan unless the old man acts, but even this threat doesn’t shake his resolve. But then Malcolm, who doesn’t really understand what’s happening, offers to give the old man his Cowboy Gonk if he’ll make the bad snowmen go away, and somehow the impractical design of the toy gets through to him. The Gonk represents a living being which could never survive in the real world, and thus represents a feat of the imagination -- an act of creativity, an aspect of the human race which makes it worth preserving. For the first time, Susan’s grandfather sees the positive possibilities in interference, and he thus agrees to save the human race. As the Box exists in several higher dimensions, it’s possible to encircle the Cold through Time and entrap it in a split second -- with the help of Malcolm, a child who still has the ability to see things as other than what they are, and who can thus provide the old man’s device with the motivational power it needs. The Cold attacks him, but John, Gillian and Zack provide the device with just enough extra power to complete its work, drawing in the animus of the Cold and trapping it within a lump of ice. Susan’s grandfather then takes the ice to Pluto in the far future. The Cold has a new and better home, the thaw has begun, and for the first time since leaving Home, Susan’s grandfather truly feels free.

There will be no school for the rest of the week, and even afterwards, things will never be the same. Gillian is now running with Zack, who can protect her from her father, but neither of them are speaking to Susan, whose presence represents things they don’t want to think about. John is also fascinated but disturbed by the truth about her, and he’s leaving London in any case, to live with his mother. Susan is upset but understands their fear; her grandfather may claim that the Cold was simply ridding its world of an infestation, but it felt personal when it was hunting them through the streets. In any case, her grandfather seems to have cleared the blocks in his memory; perhaps because the rule of meddling was more deeply imprinted in his mind, breaking that rule has cleared more of his memories than it has Susan’s. Perhaps this is why he allowed her to go out into the human world in the first place, in order to learn about it and make the decisions which he couldn’t -- in which case, considering Francis Minto, the rude soldiers and the Haighs, the human race may owe its existence to the fact that Gillian was nice to the new girl on her first day at school. The Box has been damaged by its journey with the Cold, and Susan’s grandfather sets to work repairing it, unsure whether he will be able to control his journeys in the future. He’ll have to work quickly; by meddling in history he may have attracted unwanted attention from Home, and he fears that he’ll have to be especially careful around Susan’s teachers, as the sharpest intellects on the planet must naturally belong to educators. He obviously still has a great deal to learn...