The narrative voice for the novel alternates between the third person and the first person, the latter in the form of a diary kept by Dr. Theodore "Theo" Faron, an Oxford don.
The novel opens with the first entry in Theo's diary. It is the year 2021; but the novel's events have their origin in 1995, which is referred to as "Year Omega". In 1994, the sperm count of human males plummeted to zero, and mankind now faces imminent extinction. The last people to be born are now called "Omegas". "A race apart", they enjoy various prerogatives. Theo writes that the last human being to be born on Earth has been killed in a pub brawl.
In 2006, Xan Lyppiatt, Theo's rich and charismatic cousin, appointed himself Warden of England in the last General Election. As people have lost all interest in politics, Lyppiatt abolishes democracy. He is called a despot and tyrant by his opponents, but officially the new society is referred to as egalitarian.
Theo is approached by a woman called Julian, a member of a group of dissidents calling themselves the Five Fishes. He meets with them at an isolated church. Rolf, their leader and Julian's husband, is hostile; but the others–Miriam (a former midwife), Gascoigne (a man from a military family), Luke (a former priest), and Julian–are more personable. The group wants Theo to approach Xan on their behalf and ask for various reforms, including a return to a more democratic system. During their discussions, as Theo prepares to meet with Xan, the reader learns how the UK is in 2021:
Theo's meeting, which turns out to be a meeting with the full Council of England, does not go well. Some of the members resent him because he resigned as Xan's advisor rather than share the responsibility of governing the UK. Xan guesses that Theo's suggestions came from others and makes clear to Theo that he will take action against dissidents.
The Five Fishes distribute a leaflet detailing their demands. The secret police visit Theo. He sees Julian in the market shortly afterwards. He tells her of the SSP visit, then tells her that if ever she needs him she only has to send for him. That night, however, Theo decides to leave England for the summer and visit the continent before nature overruns it.
Soon after Theo's return, Miriam tells him that Gascoigne was arrested as he was trying to rig a Quietus landing stage to explode. The other Fishes are about to go on the run, and Julian wants him. Miriam reveals why Julian did not come herself—she is pregnant. Theo believes Julian is deceiving herself, but when the two meet, Julian invites Theo to listen to her baby's heartbeat.
During the group's flight, Luke is killed while trying to protect Julian during a confrontation with a wild gang of Omegas. Julian confesses that the father of her child is not Rolf, but rather the deceased Luke. Rolf, who believes he should rule the U.K. in Xan's place, is angered at the discovery; he abandons the group to notify the Warden.
The group heads to a shack Theo knows of. Miriam delivers Julian's baby– a boy, not a girl as Julian had thought. Miriam goes to find more supplies; after she is gone too long, Theo investigates. He finds Miriam dead, garrotted in a nearby house. Theo returns to Julian, but soon afterward Julian hears a noise outside – Xan.
Theo and Xan confront each other and both fire one shot. The sudden wailing of the baby startles Xan, causing him to miss, as Rolf had thought the baby would not be born for another month. Theo shoots and kills Xan. He removes from Xan's finger the Coronation Ring, which Xan had taken to wearing as a symbol of authority, and seems poised to become the new leader of the UK– at least temporarily. The other members of the Council are introduced to the baby, and Theo baptises him.