A stark opening montage introduces a world undone by pollution and a virulent new disease that attacks every member of the grass family—wheat, rice, and maize—and unleashes widespread hunger and chaos across continents. The crisis spirals into famine, anarchy, and even cannibalism as societies crumble. In a desperate bid to survive, the Chinese unleash nerve gas on their own people, killing 300 million, a brutal gambit born of panic and an attempt to secure scarce resources.
A year into the disaster, John Custance, his family, and their daughter’s boyfriend, scientist Roger Burnham, flee riot-torn London just as roadblocks begin to form. They head north to the farm of his brother, David Custance, hopeful that a more remote valley might offer safety and a chance to rebuild something lasting.
Their escape includes a tense stop at the shop of a stubborn shopkeeper, Mr. Sturdevant. When permits are demanded, John and Roger overpower him, only to be restrained by his assistant, Andrew Pirrie. After John explains the danger they face, Pirrie shoots Sturdevant and himself and wife Clara join the party, sensing a possible protector in the new group. The decision to travel together hardens into a fragile alliance as they press on, hoping to outrun roving gangs and violent factions.
Past a brutal Army roadblock, the group is forced to shoot three soldiers to keep moving. A rivalry between Roger and Pirrie sends them racing in separate cars, and John is halted by a gate at a train crossing. He is knocked out, and his wife Ann Custance and his 16-year-old daughter Mary Custance are abducted and raped by three men. John and the others find them and manage to kill two of the attackers, though one escapes into the wreckage of the world they’re crossing.
The party keeps moving, only to be halted by vigilant locals guarding a settlement. They are robbed of their remaining resources before being forced to push onward on foot. Fifty miles from their destination, they stumble on an isolated farmhouse and, in a grim turn, kill the farmer and his wife to take their guns. In an abandoned factory, Pirrie’s wife Clara—desperate for protection—tries to seduce John and is shot by her husband in a brutal moment of tension and fear.
As trust frays, Mary Custance grows closer to Pirrie, believing he can keep her safe in a world where danger lurks at every turn. John’s party then encounters a larger, despairing group heading the opposite way, and he offers to take them to his brother’s valley. The group’s leader balks, reaching for a weapon, but Pirrie shoots him, and the others decide to join John’s march toward safety.
A motorcycle gang wheels past, and John’s wife Ann recognizes one of the riders as a former rapist from the earlier incident. The marauders attack in a series of mounted assaults, leaving casualties on both sides as the gunfights claim several of John’s people and some of the attackers.
When they finally reach David’s valley, the valley’s defenses—stone walls and a machine gun—promise safety, but David privately confesses that he cannot admit such a large crowd to his protected refuge. He suggests John slip away with his family and Roger in the night. Instead, John launches a night assault to seize the valley for his own group. In the ensuing struggle, Pirrie shoots David at the machine gun, and Pirrie himself is killed in the melee. The attack succeeds, and John assumes leadership of the valley, determined to defend and steward the fragile haven they managed to secure in a collapsed world.
