Trama

Thirteen-year-old fledgling writer Briony Tallis irrevocably changes the course of several lives when she accuses her older sister's lover of a crime he did not commit.

When Briony Tallis (Saoirse Ronan), thirteen-years-old and an aspiring writer, sees her older sister Cecilia (Keira Knightley) and Robbie Turner (James McAvoy) at the fountain in front of the family estate, she misinterprets what is happening, thus setting into motion a series of misunderstandings and a childish pique that will have lasting repercussions for all of them. Robbie is the son of a family servant toward whom the family has always been kind. They paid for his time at Cambridge and now he plans on going to medical school. After the fountain incident, Briony reads a letter intended for Cecilia and concludes that Robbie is a deviant. When her cousin Lola (Juno Temple) is raped, she tells the Police that it was Robbie she saw committing the deed.—garykmcd

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.—Huggo

The day that thirteen-year-old Briony Tallis (Saoirse Ronan) came across her older sister Cecilia (Keira Knightley) and her lover Robbie Turner (James McAvoy) making love in the library was a day that would have repercussions for all three of them. Briony misunderstands what has taken place between them and her reaction later tears the two lovers apart perhaps forever as war breaks out in Britain.—mel47

In 1935, Briony Tallis (Saoirse Ronan), a 13-year-old girl from a wealthy English family, has just finished writing a play. As Briony attempts to stage the play with her cousins, they get bored and decide to go swimming. The cousins include Lola, Leon (Patrick Kennedy) and the twin young boys Pierrot & Jackson. Danny was a worker at the estate.

Briony stays behind and witnesses a significant moment of sexual tension between her older sister, Cecilia (Keira Knightley), and Robbie Turner (James McAvoy), a servant's son, a man that Briony has a childish crush on. During Cecilia's and Robbie's argument by the garden fountain, Robbie accidentally breaks a vase (whose pieces fall on the ground and one of them into the fountain) and yells at Cecilia to stay where she is to avoid cutting her feet. Still angered, Cecilia strips off her outer clothing and climbs into the fountain to retrieve one of the glass pieces. Briony observes from the window and misinterprets the relationship between Cecilia and Robbie. Briony saw them from a great distance and could not hear a single word and assumes that Robbie commanded Cecilia to disrobe and get wet in the fountain water.

Robbie's mother is a housekeeper at the estate and his father ran away 20 years ago. The Lord of the Manor sponsored Robbie's studies when he selected to Cambridge, where he studied at the same time as Cecilia. Robbie plans to study further to become a doctor.

Robbie returns home and writes several drafts of letters to Cecilia, to apologize for the incident, including one that is explicit and erotically charged. He does not, however, intend to send it and sets it aside. Although he never intends for anyone to see this version of the note, he mistakenly places it (instead of the second more formal note he drafts) in an envelope. On his way to join the Tallis family celebration, Robbie asks Briony to deliver his letter, only to later realize that he has mistakenly given her the prurient draft. Briony reads the letter before giving it to her sister. Later, she describes the note to her 15-year-old cousin, Lola, who calls Robbie a "sex maniac". Briony is simultaneously disgusted and jealous.

Paul Marshall, a visiting friend of Briony's older brother Leon, introduces himself to the visiting cousins and appears attracted to Lola. Paul has a Chocolate factory and has a contract to supply the British Army. That evening, Cecilia and Robbie meet in the library, where they make love and then confess their love for one another. During the act, Briony watches through the partially open door and her confused emotions about Robbie become heightened.

At dinner it is revealed that the twin cousins have run away. Briony goes off alone into the woods looking for them and stumbles upon a man running away from apparently sexually assaulting her teenage cousin Lola (Juno Temple). Lola claims that she does not know the identity of her attacker. In a fit of pique, the still-hurt Briony tells everyone, including the police, that she saw Robbie commit the act. She shows Robbie's shocking letter to her mother Emily (Harriet Walter). Everyone believes her story except for Cecilia and Robbie's mother Grace Turner (Brenda Blethyn). Later, Robbie, who finds the twins unharmed, returns to the house. He is arrested despite Cecilia's pleas of his innocence. Robbie is sent to prison.

Four years later, Robbie is released from prison on condition that he join the army. He is assigned to A Company, 1st Battalion, Royal Sussex Regiment. He is reunited with Cecilia (who has not spoken with her family since the incident) in London, where they renew their love before he is shipped off to the French front.

Briony (Romola Garai), now 18, has joined Cecilia's old nursing corps at St. Thomas's in London because she wants to be of some practical use to society and has given up an offer she received from Cambridge. Her attempts at contacting her sister go unanswered, as Cecilia blames her for Robbie's imprisonment.

Separated from his unit, Robbie makes his way on foot to Dunkirk. He thinks back to six months earlier when he met Cecilia, now a nurse. Robbie, who is gravely ill from an infected wound and hallucinating, arrives at the beaches of Dunkirk, where he awaits evacuation.

Later, Briony, who regrets implicating Robbie, learns that Paul Marshall is about to marry Lola. As Briony attends the wedding, it sinks in that Paul assaulted Lola during the search for the twins years ago. Briony, now fully understanding the consequences of her accusation, visits Cecilia to ask forgiveness and suggests correcting her testimony, to which Cecilia says she would be an "unreliable witness". Briony is surprised to find Robbie living with her sister while in London on leave.

Cecilia coldly replies that she will never forgive her. Briony apologizes for her deceit, but Robbie is enraged that she has not accepted responsibility for her actions. Cecilia calms him down, and Robbie instructs Briony how to set the record straight and get his conviction overturned. Briony agrees. Cecilia adds that Briony should include what she remembers of Danny Hardman (Alfie Allen) (a worker on the estate), but Briony points out that Paul Marshall (Benedict Cumberbatch) was the person who assaulted Lola and now Lola will not be able to testify against her husband.

when Briony (Vanessa Redgrave) is an elderly and successful novelist, she gives an interview about her latest and last book, an autobiographical novel titled Atonement, and explains that she is dying from vascular dementia.

Briony says that her novel, which she has been working on for most of her adult life, will be her last.

Briony reveals that the portion of the book where Robbie and Cecilia are living together and Briony attempts to apologize to them is fictitious. The reality is that she could never atone for her mistake, and Cecilia and Robbie never reunited.

Robbie died of septicemia from his infected wound at Dunkirk, and Cecilia drowned months later during an underground flood due to the Balham tube station bombing during the Blitz.

Briony admits that she wrote her novel with its fictitious ending to give the two, in fiction, the happiness they never had because she was responsible for mistakenly identifying Robbie as Lola's sexual assault perpetrator. An imagined Cecilia and Robbie happily stay together in the house by the sea which they had intended to visit once they were reunited.