One September afternoon in 1999, teenagers Matthew, Zoe, and Duncan Lang are walking home from school when they discover a boy lying in a field, bloody and unconscious. Thanks to their intervention, the boy's life is saved. In the aftermath, all three siblings are irrevocably changed.
Matthew, the oldest, becomes obsessed with tracking down the assailant, secretly searching the local town with the victim's brother. Zoe wanders the streets of Oxford, looking at men, and one of them, a visiting American graduate student, looks back. Duncan, the youngest, who has seldom thought about being adopted, suddenly decides he wants to find his birth mother.
Overshadowing all three is the awareness that something is amiss in their parents' marriage. Over the course of the autumn, as each of the siblings confronts the complications and contradictions of their approaching adulthood, they find themselves at once drawn together and driven apart.