On the hottest day of the summer of 1935, thirteen-year-old Briony Tallis sees her older sister Cecilia (Kiera Knightley) strip off her clothes and plunge into the fountain in the garden of their country house. Watching Cecilia is their housekeeper's son Robbie Turner (James McAvoy), a childhood friend who, along with Briony's sister, has recently graduated from Cambridge.

By the end of that day the lives of all three will have been changed forever. Robbie and Cecilia will have crossed a boundary they had never before dared to approach and will have become victims of the younger girl's scheming imagination, and Briony will have committed a dreadful crime, the guilt for which will colour her entire life.