In 2027, after 18 years of global human infertility and depression, the world is on the brink of collapse and humanity faces extinction. The United Kingdom, one of the few nations with a functioning government (the only other known functioning government is Angola), is deluged by asylum seekers fleeing radiation and plague. In response, the UK has become a police state as the British Army rounds up and executes immigrants. Theo Faron, a former activist turned cynical bureaucrat, is kidnapped by the Fishes, a militant immigrants’ rights group. They are led by Theo’s estranged wife, Julian Taylor, from whom he separated after their son’s death during a 2008 flu pandemic.
Julian offers Theo money to acquire transit papers for a young refugee, Kee. Theo obtains the papers from his cousin, a government minister who runs a state-sponsored collection of salvaged art. Theo agrees to escort Kee in exchange for a large sum. Luke, a Fishes member, drives Theo, Kee, and former midwife Miriam towards Canterbury. They are ambushed by an armed gang, and Julian is killed. When the group is stopped by the police, Luke kills the officers and the group hides Julian’s body before heading to a Fishes safehouse.
Kee reveals to Theo that she is pregnant. Julian had intended to hand Kee to the Human Project, a supposed scientific group in the Azores dedicated to curing infertility. However, Luke persuades Kee to stay and is voted as the new leader of the Fishes. That night, Theo eavesdrops on a discussion and learns that Julian’s death was orchestrated by the Fishes so that Luke could become leader; they intend to kill Theo and use the baby as a political tool to support the coming revolution. Theo wakes Kee and Miriam, and they steal a car, escaping to the secluded hideaway of Theo’s ageing friend Jasper Palmer, a former political cartoonist turned pot dealer.
The group makes plans to board the Human Project ship, the Tomorrow, which will arrive offshore from a refugee camp at Bexhill-on-Sea. Jasper proposes having Syd, a camp border guard to whom he frequently sells drugs, smuggle them into Bexhill, masquerading as refugees. When the Fishes discover Jasper’s house, the group flees while Jasper stays behind to stall the Fishes. Luke shoots and kills Jasper as Theo watches from the woods. The group meets Syd at an abandoned school, and he helps them board a bus to Bexhill. When Kee experiences contractions at a checkpoint, Miriam distracts a guard by feigning religious mania and is taken away.
Inside the camp, Theo and Kee meet a Romanian woman, Marichka, who provides a room where Kee gives birth to a baby girl. The next day, Syd informs Theo and Kee that war has broken out between the British military and the refugees, led by the Fishes. Having learned that they have a bounty on their heads, Syd attempts to capture them, but Theo kills him with a car battery and they escape. Amidst the fighting, the Fishes capture Kee and the baby. Theo tracks them to an apartment building under heavy fire; he confronts Luke, who is killed in an explosion, and escorts Kee and the baby out. Awed by the baby, the British soldiers and Fishes temporarily stop fighting and allow the trio to leave. Marichka leads them to a hidden boat, but stays behind as they depart, despite Theo imploring her to come along.
As British fighter jets bomb Bexhill from a distance, Theo reveals that he was mortally wounded by Luke. He tells Kee how to soothe her baby, and Kee tells Theo she will name her Dylan after Theo and Julian’s lost son. Theo dies as the Tomorrow approaches and Kee tries to wake him to see that they’ve been saved.
As the credits roll, the sound of children laughing and playing can be heard in the background.