In 2027, after 18 years of global human infertility, civilization is on the brink of collapse as humanity faces extinction. The United Kingdom, the only stable nation with a functioning government, is deluged by asylum seekers fleeing the chaos and war which have taken hold around the world. In response, Britain has become a militarized police state as British government forces round up and detain 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.
Julian offers Theo money to acquire transit papers for a young refugee named Kee, which Theo obtains from his cousin Nigel, a government minister. The bearer must be accompanied, so Theo agrees to escort Kee in exchange for a large sum. Luke, a Fishes member, drives them and former midwife Miriam towards the coast to a boat. They are ambushed by an armed gang and Julian is killed. Luke kills two police officers who stop their car and they escape to a safe house.
Kee reveals to Theo that she is the only known pregnant woman on Earth. Julian had told her to only trust Theo, intending to hand Kee to the "Human Project", a supposed scientific group in the Azores dedicated to curing infertility. However, Luke persuades Kee to stay. That night, Theo eavesdrops on a meeting of Luke and other Fishes, and discovers that Julian's death was orchestrated by the Fishes so Luke could take over as leader, and the Fishes could 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 aging friend Jasper Palmer.
The group plans to board the Human Project ship Tomorrow, which will arrive offshore from the Bexhill refugee camp. Jasper proposes getting Syd, a camp guard he knows, to smuggle them. The Fishes trail the group and Jasper stays behind to stall them, giving a government-issued suicide drug to his wife before being killed by the Fishes. The group meets with Syd, who transports them to Bexhill disguised as prisoners. When Kee's contractions begin on a bus, Miriam distracts a suspicious guard by feigning mania and is taken away.
At the camp, Theo and Kee meet a Romani woman, Marichka, who provides a room where Kee gives birth to a girl. The next day, Syd finds them in their room and informs Theo and Kee that war has broken out between the British Army and the refugees, including the Fishes. Having learned that they have a bounty on their heads, he attempts to capture them, but Marichka and Theo fight him off and the group escapes. Amidst the fighting, the Fishes capture Kee and the baby. Theo tracks them to an apartment under heavy fire from the military and escorts her out. Awed by the baby, the combatants temporarily stop fighting. Marichka leads Theo, Kee, and the baby to a boat in a sewer and Theo rows away. As they watch the bombing of Bexhill by the Royal Air Force from a distance, Theo reveals that he has been shot. He dies from his wounds as the Tomorrow approaches through the fog.