Director Joe Wright re-teams with Keira Knightley for an adaptation of Ian McEwan's 2001 novel, a story that traverses a good portion of the 20th Century as it explores the devastating impact caused by rash decisions and the costs associated.
The film opens in Summer 1935 at the Tallis estate as the family prepares for the return of eldest child Leon accompanied by his friend Paul (Benedict Cumberbatch). A young teenage Briony (Saoirse Ronan) is writing a play for the occasion and subjects her cousins, a girl about her age and two younger twin boys who are staying because their parents are going through a divorce, to rehearse it. Recently returned from college are Briony's older sister, Cecilia (Knightley) and Robbie Turner (James McAvoy), the son of the Tallis' housekeeper (Brenda Blethyn). Turner's fees were paid by the Tallis patriarch and will continue to be as Robbie heads off to medical school.
Briony notices uneasiness between Cecilia and Robbie. From the house, she only can see an incident at the fountain where Cecilia took off some clothing in front of Robbie and jumped in. Later, Robbie asks Briony to deliver a note to Cecilia. Naturally, the curious teen reads it and is shocked to find Robbie's sexual desires as well as his word choices.
Wright jumps back in the narrative, revealing events so the audience becomes more privy than Briony to the true relationship between Cecilia and Robbie. We see and hear their exchange at the fountain and learn what caused Cecilia to jump in. We see Robbie sent the wrong letter. The true nature of their relationship is made clear so the audience isn't as surprised by Cecilia and Robbie having sex in the library as Briony is when she stumbles upon them.
During Leon's dinner, the twins are discovered to have run away and the meal is put on hold as everyone searches for him on the estate. Making her way through the darkness, Briony is startled by a man running off as he leaves behind cousin Lola (Juno Temple) who has been raped. Lola can't identify the man, saying her eyes were covered, but with what Briony has witnessed recently she is certain it was Robbie. She informs her mother and the police that she saw Robbie rape Lola and uses his letter to Cecilia as an example of his depravity. Although he marches in triumphant with the twins in two, he is promptly arrested.
The story moves ahead to 1940. Robbie is a private in the British army, as prisoners were allowed out to serve. He and fellow soldiers from his unit are in France heading to Dunkirk to be evacuated. Both Cecilia and Briony (played at this age by Romola Garai) are serving as nurses in London, though Cecilia ignores Briony's requests to see her. While talking with her friend Fiona, a flashback shows an incident between Briony and Robbie that better clarifies their relationship and her actions.
Briony goes to Cecilia's apartment where she begs forgiveness of her sister and Robbie. As the couple's anger briefly quells, they demand Briony inform the family and the authorities.
The story leaps again to 1999 where Briony (played at this age by Vanessa Redgrave) is a successful author being interviewed about her latest novel, Atonement. Because of illness, she states it will be her last but points out it really is her first novel as she has been working on it for ages. With the book she hopes, as inadequately as it will be, to make up for the years of heartache and pain she caused those closest to her and to herself.