The book follows Henry Perowne, a middle-aged, successful surgeon. Five chapters chart his day and thoughts on Saturday the 15 February 2003, the day of the demonstration against the 2003 invasion of Iraq, the largest protest in British history. Perowne's day begins in the early morning, when he sees a burning aeroplane streak across the sky. This casts a shadow over the rest of his day as reports on the television change and shift: is it an accident, or terrorism?
En route to his weekly squash game, a traffic diversion reminds Perowne of the anti-war protests occurring that day. After being allowed through the diversion, he collides with another car, damaging its wing mirror. At first the driver, Baxter, tries to extort money from him. When Perowne refuses, Baxter and his two companions become aggressive. Noticing symptoms in Baxter's behaviour, Perowne quickly recognises the onset of Huntington's disease. Though he is punched in the sternum, Perowne manages to escape unharmed by distracting Baxter with discussions of his disease.
Perowne goes on to his squash match, still thinking about the incident. He loses the long and contested game by a technicality in the final set. After lunch he buys some fish from a local fishmonger for dinner. He visits his mother, suffering from vascular dementia, who is cared for in a nursing home.
After a visit to his son's rehearsal, Perowne returns home to cook dinner, and the evening news reminds him of the grander arc of events that surround his life. When Daisy, his daughter, arrives home from Paris, the two passionately debate the coming war in Iraq. His father-in-law arrives next. Daisy reconciles an earlier literary disagreement that led to a froideur with her maternal grandfather; remembering that it was he who had inspired her love of literature. Perowne's son Theo returns next.
Rosalind, Perowne's wife, is the last to arrive home. As she enters, Baxter and an accomplice 'Nige' force their way in armed with knives. Baxter punches the grandfather, intimidates the family and orders Daisy to strip naked. When she does, Perowne notices that she is pregnant. Finding out she is a poet, Baxter asks her to recite a poem. Rather than one of her own, she recites Dover Beach, which affects Baxter emotionally, effectively disarming him. Instead he becomes enthusiastic about Perowne's renewed talk about new treatment for Huntington's disease. After his companion abandons him, Baxter is overpowered by Perowne and Theo, and knocked unconscious after falling down the stairs. That night Perowne is summoned to the hospital for a successful emergency operation on Baxter. Saturday ends at around 5:15 a.m. on Sunday, after he has returned from the hospital and made love to his wife again.