In 2027, eighteen years after human activities have caused widespread ecocide, total human infertility, war, and global depression threaten the collapse of human civilizations.[11] The United Kingdom is one of the few remaining nations with a functioning government and has become a police state in which immigrants are arrested and either imprisoned or executed.
Theo Faron, a former activist turned cynical bureaucrat, is kidnapped by the Fishes, a militant immigrant-rights group led by Theo's estranged wife, Julian Taylor; the pair separated after their son's death during a 2008 flu pandemic. Julian offers Theo money to acquire transit papers for a young refugee woman named Kee. Theo obtains the documents from his cousin, a government minister, and agrees to escort Kee in exchange for a larger sum of money. Luke, a Fishes leader, drives Theo, Kee, Julian, and former midwife Miriam towards Canterbury, but an armed gang ambushes them and kills Julian. Two police officers later stop their car; Luke kills them, and the group hides Julian's body before heading to a safe house.
Kee reveals to Theo that she is pregnant, making her the only known pregnant woman in the world. Julian had intended to take her to the Human Project, a scientific research group in the Azores dedicated to curing humanity's infertility, which Theo believes does not exist. Luke becomes the new leader of the Fishes. That night, Theo eavesdrops on a discussion and learns that the Fishes orchestrated Julian's death so that Luke could become their leader and that they intend to kill him and use Kee's baby as a political tool. Theo wakes Kee and Miriam, and they escape to the secluded hideaway of Theo's reclusive, aging hippie friend Jasper Palmer, a former political cartoonist whose wife was tortured into catatonia by the British government for her activism.
The group plans to reach the Human Project ship, the Tomorrow, scheduled to arrive offshore at Bexhill, a notorious immigrant detention centre. Jasper plans to use Syd, an immigration officer to whom Jasper sells cannabis, to smuggle them into Bexhill as refugees, from where they can take a rowing boat to rendezvous with the Tomorrow. The next day, the Fishes discover Jasper's house, forcing the group to flee. Jasper stays behind to stall them and is murdered by Luke as Theo watches. Theo, Kee, and Miriam meet with Syd, who helps them board a bus to the camp. When Kee begins experiencing contractions, Miriam distracts a guard by feigning religious mania and is taken away.
Inside the camp, Theo and Kee meet a Romani woman, Marichka, who provides a room where Kee gives birth to a baby girl. The next day, Syd tells Theo and Kee that war has broken out between the British military and the refugees and that the Fishes have infiltrated the camp; he then reveals that Theo and Kee have a bounty on their heads and attempts to capture them. Theo subdues Syd with Marichka's help; they escape but are ambushed by the Fishes, who capture Kee and the baby. Theo tracks them to an apartment building that is under heavy fire. Theo confronts Luke, who is killed in an explosion, and Theo 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 the boat but stays behind as they depart.
As British fighter jets conduct airstrikes on Bexhill, Theo and Kee row to the rendezvous point in heavy fog. Theo reveals that he was shot and wounded by Luke earlier; he teaches Kee how to burp her baby, and she tells him she will name the baby girl Dylan, after Theo's and Julian's lost son. Theo smiles weakly, then loses consciousness as the Tomorrow approaches. As the screen cuts to black, children's laughter is heard.
