Saturday follows a day in the life of Henry Perowne, a successful neurosurgeon and father to a comfortable middle-class family. The day is the 15th of February 2003 – the same day as a large protest across Britain against Tony Blair’s support for the American invasion of Iraq.
The novel begins in the early hours of the morning as Perowne wakes for the day. In his first thoughts and reflections of the day are glimpses of his work and family life: he recalls the technical brilliance of the surgeries he has completed in the last week, and makes reference to his lawyer wife, Rosalind, their prize-winning poet daughter, Daisy, and guitarist son, Theo. As Perowne stretches and walks towards his bedroom window, he catches sight of a burning plane crashing down towards Heathrow. He debates whether he has just witnessed an act of terrorism or simply an unfortunate accident. Reports and updates on the plane crash appear in fragments throughout the course of the rest of the novel.
As Henry drives towards his weekly game of squash with his colleague, he thinks about the protest taking place later that day. A collision with another car, damaging its wing mirror, leads Perowne into an argument with a driver named Baxter. After Perowne refuses to give Baxter and his two thuggish companions money, the argument takes a violent turn. Observing Baxter’s character, Perowne diagnoses him with the early stages of Huntington’s disease and manages to escape the fight by distracting Baxter with this information. Perowne, distracted by these events, loses his game of squash to his colleague, Jay Strauss. Perowne and Strauss also discuss the protest, with Perowne liking to take the position of devil’s advocate.
After he buys fish for the family dinner that evening – which is to be attended by his father-in-law, John Grammaticus, a poet – he visits his widowed mother, Lily, who is in a care home due to the onset of vascular dementia. Perowne also visits Theo at one of his rehearsals before returning home.
When Perowne starts to cook dinner at home, he argues with Daisy – back from a trip to Paris and also back from attending the protest earlier in the day – about the impending war. When John arrives, Daisy and John reconcile a literary argument they had previously had. Theo then arrives, later followed by Rosalind.
When Rosalind gets home, things take a turn for the worse as she is accompanied by Baxter and one of his companions, holding her at knife point. Baxter takes a swipe at John, leaving him with a broken nose. He then instructs Daisy to take her clothes off. When she strips, the family come to the shocking realization that Daisy is pregnant. As Baxter talks to Daisy, she reveals she is a poet and he asks her to recite a poem for everyone. She reads 'Dover Beach' by Matthew Arnold. Surprisingly, Baxter is so emotionally affected by the poem that Perowne is able to distract him by talk of treatment for Huntington’s. Perowne and Theo manage to keep Baxter from any further violence, and Baxter is left unconscious after falling down the stairs.
Perowne is called into the hospital to complete an emergency operation on Baxter. The novel ends on the Sunday morning when Perowne returns from the hospital and gets into bed with his wife Rosalind, 24 hours from when the novel started.
