In 1935, 13-year-old Briony Tallis discovers the burgeoning relationship between her older sister Cecilia and Robbie Turner, a servant's son whom she has a crush on herself. The teenager becomes increasingly jealous of the pair and spitefully accuses him of a crime he did not commit, unaware her actions will have tragic repercussions. Joe Wright's romantic period drama, based on Ian McEwan's novel, starring Keira Knightley, James McAvoy, Brenda Blethyn, Saoirse Ronan and Romola Garai