The year is 1935 and on a hot summer’s day a chain of events set in motion. Briony Tallis (Saoirse Ronan) is a young, imaginative preteen writer. She is the younger sister of Cecilia (Keira Knightley) who is an attractive, Cambridge graduate in an upper class family enjoying her life. On this hot summer’s day she decides to take a swim in the family fountain. Robbie Turner (James McEvoy), the son of the housekeeper watches the beauty as she goes for a swim and little Briony’s imagination runs rampant.

It is not long until a few misconceptions and Briony’s wild imagination starts rumours which lead to the arrest and conviction of Robbie; An event that will dramatically change the lives of all three people.

Five years later in the madness of World War II appears Robbie, as a foot soldier in the army. He is preparing for an evacuation of Dunkirk. Meanwhile the two sisters train as nurses in London, all deeply affected by the events years before.