In 1939, Robbie Turner (James McAvoy) is a Private in the British Army, he and his battalion who are heading to France to fight in the war. A stint in the army is not where his life was headed, which took a radical turn four years earlier at the Tallis estate where he grew up as his mother worked as the Tallis’ live-in housekeeper. As such, he grew up with the three Tallis children: son Leon (Patrick Kennedy), and daughters Cecilia (Keira Knightley) and Briony (Saoirse Ronan). Robbie and Cecilia were just getting to the stage of their lives of being able to confess their true love for each other. But then thirteen-year-old aspiring writer Briony also had a crush on the older Robbie. Based on two incidents she saw between Cecilia and Robbie (one only from afar), on reading a private letter Robbie wrote to Cecilia, and on her own feelings for Robbie, Briony told some truths and half-truths about Robbie which resulted in this turn in his life. Robbie is able to reconnect with Cecilia before he is shipped off to France, and lives only to be able to head back to London to be with Cecilia and make up for the missing four years they could not spend together due to Briony’s actions. A year later, Briony (Romola Garai), now eighteen, also arrives in London to start working as a nurse to support the war effort. By this time, she is aware of the damage her thirteen-year-old self caused, and wants to atone for that error to Cecilia, from whom she has been estranged, and Robbie, who she has not seen since. She will make this atonement even if neither Robbie or Cecilia will ever speak to her, or if it takes the rest of her life.