Jimmy and Bob emerge from under the truck to see a mushroom cloud rising over the city. Jimmy then flees through Sheffield to try to reach Ruth (this is the last we see of him, save for a later flashback); Ruth and her family hurry to their basement with Ruth's frail grandmother, leaving their cat outside; and Jimmy's parents quickly prepare a woefully inadequate shelter out of mattresses, bags and doors. Ruth suddenly decides to rush outside, but her father quickly catches her and brings her back just before a direct strike hits Sheffield city centre. The city is devastated; most of it is set ablaze, with Jimmy presumably being killed and his mother being severely burnt and partially blinded by the blast. The body of her younger son Michael is later found under the rubble of the aviary where Jimmy kept his exotic finches. Their daughter Alison, having gone to the shops beforehand, is not seen during the attack scene, but she does survive unlike her brothers. One of the chief executive's team dies from falling rubble in the bunker. On-screen text tells us that 210 megatons have fallen on the United Kingdom (with 3000 megatons total falling around the world), that two-thirds of houses are in fire zones, and that immediate deaths are between 17-30 million. In addition, the threat from fallout means no attempt is made to fight the fires or rescue those trapped by the flames. Accompanying these grim statistics is a sequence which not only shows milk bottles melting in the heat, but also shows human corpses burning and the Becketts' cat dying. Both of the families suffer from immediate
radiation sickness, though the Becketts suffer less due to being outside a fire zone. Ruth, however, fears the radiation she has been exposed to will leave her unborn child "ugly and deformed". Ruth's mother tries to convince her that Jimmy might still be alive, but Ruth says that she knows Jimmy is not. Ruth later leaves the cellar and wanders through the devastated city, while the remaining hospitals are inundated (in a particularly gruesome scene, a man has his leg amputated with only a rag in his mouth to help him). Jimmy's mother eventually dies from radiation sickness, and her husband leaves their shelter. Ruth goes to the Kemp home, finds Jimmy's mother and takes one of Jimmy's books from the ruins of the home as a keepsake. Ruth's grandmother dies in her sleep, and her parents are murdered by looters, who are then caught by soldiers; one of the looters is shot dead. Jimmy's father joins the fight to get food from a nearby warehouse, which isn't to be distributed for two weeks, and the soldiers have to use tear gas and eventually gun down any who try to break in to control the situation. Tensions rise amongst the chief executive's team as it becomes increasingly clear that they are trapped and that the situation outside the bunker is virtually beyond repair.
Jimmy's father is seen with another man swapping cigarettes for alcohol, but he is unable to keep it down. He then turns on Michael's handheld video game, which, though damaged, still works. Jimmy's father eventually is seen in a stilled black-and-white photograph as dead among the countless bodies, presumably of radiation sickness. (following this photo is a caption stating that no efforts are made to bury the bodies, and that burning them is considered a waste of fuel; millions of bodies are left unburied throughout the UK.) This leaves Alison Kemp, Jimmy's sister, as the sole survivor of the Kemp family, although she is only seen briefly through the fence of a tennis court being used as a detention center. The government authorizes the use of firing squads to execute looters without a fair trial, and presumably Alison is shot for her offense. When they realize that the food stocks are dwindling, they are forced to cut the rations to 1000 calories per day for those who can work and 500 calories per day for those who can't. In essence, "those who work get more food than those who don't, and the more people die, the more food there is for the rest." After four weeks, when the city hall rubble is finally moved to allow rescue workers to enter the bunker, all members of the team are found to be dead, presumably of asphyxiation.
Two months after the war, a radio message is heard saying that if Britain is to rebuild then it must go agricultural; however, the temperature has drastically fallen and the materials needed for modern farming - chemicals, fertiliser and fuel - are in short supply. Only those who work in the rebuilding will receive food. Ruth finds Jimmy's friend Bob, and together they find a dead sheep and eat it raw, despite its having died from radiation. Eventually, Ruth gives birth alone in a stable to her baby (while it is neither stillborn nor obviously deformed, she has to cut the
umbilical cord by biting it). She is seen later, staring vacantly as the baby cries, with a group of people around a fire on Christmas Day. One year after the war, sunlight begins to return bringing much ultraviolet light, prompting harvesters to don protective clothing. Subsequent harvests produce lesser returns due to the lack of proper equipment.
Three to eight years after the war, Britain's population is estimated to have fallen to medieval levels of 4-11 million. The country has indeed managed to reindustrialize itself to some degree (though only to a very primitive Third World level; photos of a steam engine, mill workers, and civilians cleaning up rubble are shown), and it appears that basic electricity has been re-established on a small scale, to the extent that electric lighting is fully available. Able-bodied survivors, including Ruth and her daughter, are shown manually working the fields ten years after the war. Ruth collapses to the ground, prematurely aged and with severe cataracts. In their living space, Ruth finally dies. The unnamed daughter (sometimes credited as Jane by the producers) tries to rouse her mother (revealing her lack of language skills). Ruth's daughter seems unfazed by her death and leaves her, as well as Jimmy's book, behind. Symbolically, she takes her mother's hairbrush and headscarf.
The post-war generation are emotionally and mentally stunted. They speak in a distorted, simplified version of English, using simple
telegraphic sentences years after pre-war children are capable of forming more complete sentences. This is a problem because when the older pre-war generation dies, the current generation will be responsible for re-establishing Britain as a fully functioning nation, and if they are not able to speak and comprehend proper English, all efforts of reconstruction will be rendered futile and useless. Their "education" seems to consist of silently watching a damaged videotape of the BBC children's programme
Words and Pictures as their teacher mouths along with the presenter's dialogue; it is not clear whether the damage to the tape is due to the effects of the bomb or simple over-use. Young girls like Ruth's daughter are put to work salvaging the threads from old clothing. Three years after Ruth's death, Ruth's daughter and two boys her age are caught stealing food. When they try to escape, one boy is shot dead. She and the other boy fight for the food; she is then overpowered, and presumably raped, by the boy. Later, she is seen stumbling through the rubble of a city, pregnant. She finds a makeshift hospital, which has basic electricity. The movie ends with Ruth's daughter giving birth to a stillborn (and presumably deformed) baby, and the film freezes just as she is about to scream in horror.