Saturday takes place on a single day—Saturday, February 15, 2003. Before dawn, Henry Perowne, a prominent neurosurgeon, watches a plane crashing toward London, possibly a terrorist attack. The plane foreshadows how Perowne’s day of playing squash with a colleague, shopping for a family dinner, and visiting his widowed mother will be interrupted by an automobile collision with Baxter, who is driving with two other young thugs. Perowne escapes being beaten to death by quickly diagnosing Baxter as having Huntington’s disease and offering help.

Later, Baxter and his buddy, Nigel, invade the Perowne home, where the family has gathered to celebrate the publication of daughter Daisy’s first volume of poems and to reconcile Daisy with her maternal grandfather, also a poet. Baxter forces Daisy to strip naked and read one of her poems, while he holds a knife at her attorney mother’s throat. When Daisy complies, the plot offers an epiphany by revealing that Daisy is pregnant. The stakes are now raised because violence to her body could precipitate a miscarriage and the destruction of a genuinely innocent life. Instead of desecrating one of her own poems, Daisy recites Matthew Arnold’s “Dover Beach.” Luckily, Baxter is even less familiar with poetry than Daisy’s father and seems moved by the sentimental, Victorian poem.

Baxter makes good on threats of violence by breaking the jaw of Daisy’s grandfather. Sensing the need to do something, Perowne lures Baxter upstairs to his study, where he tells him he has information about Baxter’s medical condition. After Perowne and his son’s effort to subdue Baxter leads to the invader’s tumbling down the stairs and Nigel flees, Perowne reveals himself as a dedicated professional by performing emergency surgery on Baxter before this Saturday ends. The semblance of domestic tranquillity is restored. However, like other families who have experienced burglaries or other home invasions, the Perownes are unlikely to recover their earlier sense that their space is inviolable.