Plot
In 1935 England, 13-year-old Briony Tallis, the youngest daughter of the wealthy Tallis family, spies on her older sister, Cecilia, and the housekeeper's son, Robbie Turner, from a window. Cecilia and Robbie argue by the garden fountain, during which Robbie accidentally breaks a vase. Cecilia removes her dress and climbs into the fountain to retrieve one of the broken pieces. Briony, infatuated with Robbie, observes from nearby and misinterprets the relationship between them.
Robbie writes Cecilia a note in apology. In one draft, he graphically describes his attraction to her, and unintentionally places this draft in the envelope, which he has Briony deliver to Cecilia. She reads it before giving it to her sister and is shocked by the content. Later, Briony describes the note to her 15-year-old cousin Lola, evacuated to their estate with her brothers, who calls Robbie a "sex maniac". Meanwhile, Paul Marshall, a visiting friend of Briony and Cecilia's older brother Leon, introduces himself to their visiting cousins, including Lola.
Before dinner, Robbie apologises to Cecilia for the obscene letter, but she confesses her love for him. They secretly have sex in the library, which Briony witnesses, causing her mistakenly think her sister is being raped. During dinner, Lola's twin brothers go missing, and the household organises search parties around the grounds. In the dark, Briony comes across Lola being raped by a man who flees. Despite not seeing him clearly, Briony insists it was Robbie; a confused and traumatised Lola does not dissent. After locating the twins, Robbie is arrested for the rape despite Cecilia's pleas of his innocence. Lola and Briony's testimony, along with her turning over the explicit letter, convinces everyone but Cecilia of his guilt.
Four years later, during the Second World War, Robbie is drafted from prison. Joining the army, he fights in the Battle of France. Separated from his unit, Robbie makes his way on foot to Dunkirk, thinking back to six months earlier when he met Cecilia, now a nurse. Gravely ill from an infected wound and experiencing hallucinations, he awaits evacuation.
Briony, now 18, joined Cecilia's old nursing unit at St Thomas' Hospital in London rather than go to the University of Cambridge. She writes to her sister, but Cecilia cannot forgive her for causing Robbie's arrest and conviction. Learning that Paul Marshall is about to marry Lola, Briony attends the wedding, and realises it was he – and not Robbie – who raped Lola. Briony visits Cecilia to ask forgiveness and suggests correcting her testimony, but Cecilia scornfully says she would be an "unreliable witness". Briony is surprised to find Robbie staying with her sister while on leave, and apologises for her deceit, but Robbie is enraged that she has not accepted responsibility for her actions. Cecilia calms him down, and he instructs Briony how to get his conviction overturned. Briony agrees. When Cecilia suggests that she implicate a former worker on the Tallis estate instead, Briony objects, but is reminded that Lola is now married and cannot testify against her husband.
Decades later Briony, an elderly and successful novelist, gives an interview about Atonement. The autobiographical novel, her latest, will be her last book, as she is dying from vascular dementia. She reveals that the portion of the book where Robbie and Cecilia are living together and Briony begs their forgiveness is fictitious. In reality she was never able to atone for her mistake, and Cecilia and Robbie never reunited; Robbie died of septicaemia from his infected wound at Dunkirk, and Cecilia drowned months later during an underground flood in the Balham tube station bombing during the Blitz. She wrote her novel with its happy ending to give the two, in fiction, the happiness they never had on her account. The last scene shows an imagined Cecilia and Robbie together in the house by the sea, which they once intended to visit when they were reunited.
