Part one
Briony Tallis, a 13-year-old English girl with a talent for writing, lives at her family's country estate with her parents Jack and Emily Tallis, who are members of the landed gentry. Her older sister Cecilia has recently graduated from the University of Cambridge with Robbie Turner, the Tallis family housekeeper's son and Cecilia's childhood friend, whose university education was funded by Jack.
In the summer of 1935, Briony's maternal cousins, 15-year-old Lola and 9-year-old twins Jackson and Pierrot, visit the family amidst their parents' divorce. Briony and Cecilia's older brother Leon returns from London, accompanied by his friend from Oxford, the well-off manufacturer Paul Marshall. Cecilia and Robbie bicker over a vase, which breaks and falls into a pond. Cecilia strips to her underwear and dives in to retrieve the pieces, surprising Robbie. Briony, watching from a window, is confused and intrigued by Cecilia and Robbie's actions. She is inspired to begin writing psychological realism, and the reader is informed that this will eventually become a hallmark of her fiction.
In the wake of the incident by the pond, Robbie realizes he is attracted to Cecilia, and writes several drafts of a love letter to her. He gives the letter to Briony to deliver to Cecilia; however, he inadvertently gives her a version he had meant to discard, which contains lewd references ("In my dreams I kiss your cunt"). By the time Robbie realizes his mistake, Briony has already returned to the house with his letter.
Despite Robbie's instructions to the contrary, Briony opens the letter and reads it. She is shocked by its vulgar language, and becomes convinced that Robbie intends to harm Cecilia. An injured Lola goes to Briony for comfort, claiming that her younger brothers attacked her, although it is implied to have instead been Paul Marshall, who has a long scratch on his face. Briony relays the contents of the letter to Lola, who labels Robbie a "maniac," re-affirming Briony's feelings. Robbie arrives at the main house for a family dinner party, and is confronted by Cecilia. He confesses his feelings to her, and she responds in kind. Later the same evening, Briony walks in on Robbie and Cecilia having sex in the library. The immature Briony believes she interrupted a vicious assault on Cecilia, and stands stunned while Robbie and Cecilia quickly exit.
At the dinner, which is generally tense, it is discovered the twins have run away. The party breaks into teams to search for them. When Cecilia goes with Leon, Robbie and Briony each set off on their own. In the darkness, while everyone is searching for the twins, Briony discovers her cousin Lola being raped by an assailant neither girl can clearly see. The attacker flees. Briony, convinced that it must have been Robbie, gets Lola to agree that she likely heard Robbie's voice. The girls return home, and Briony identifies Robbie to the police as the rapist, claiming she saw his face in the dark. Lola is sedated by the local doctor, Cecilia screams at Briony and locks herself in her room, and Paul Marshall shares cigarettes with the policemen.
Robbie does not return, and the family and police officers stay awake waiting for him. As dawn breaks, Robbie appears in the driveway with Jackson and Pierrot, having found and rescued them. He is arrested on the spot and taken away, with only Cecilia and his mother believing his protestations of innocence. Briony is satisfied by this conclusion to her mythologized version of the events, with her as the hero and Robbie as the villain.
Part two
By the time the Second World War has started, Robbie has spent several years in prison. He and Cecilia have passed several years exchanging letters, maintaining their love for each other. Robbie is released from prison on the condition he enlist in the army. Meanwhile, Cecilia has completed training as a nurse, and cut off all contact with her family for the parts they played in locking Robbie up. Shortly before Robbie is deployed to France, they meet once for half an hour, during Cecilia's lunch break. Their reunion starts awkwardly, but they share a kiss before leaving each other.
In France, the war is going badly, and the army is retreating to Dunkirk. As the injured Robbie makes his way there, he thinks about his love for Cecilia and his hatred for Briony. However, he eventually concludes that Briony was too young to be blamed fully, and writes Cecilia a letter encouraging her to reconnect with her family. His condition deteriorates over the course of the section; he weakens and becomes delirious. Robbie falls asleep in Dunkirk, one day before the evacuation begins.
Part three
A remorseful Briony, now eighteen years old, has refused her place at Cambridge and instead is a trainee nurse in London. She has realised the full extent of her mistake and decides it was Paul Marshall, Leon's friend, whom she saw with Lola. Briony still writes fiction, mostly inspired by Robbie and Cecilia's relationship, although she has been rejected by several literary publications.
Briony travels to attend the wedding of Paul Marshall and her cousin Lola, with the knowledge that Lola is marrying her rapist. Briony considers speaking up during the wedding, but does not. Afterwards, she visits Cecilia, who is cold but invites Briony in nonetheless. While Briony is apologizing to Cecilia, Robbie unexpectedly appears from the bedroom. He has been living with Cecilia while he is on leave from the army. Robbie expresses his fury at Briony, but with Cecilia's soothing remains civil.
Cecilia and Robbie both refuse to forgive Briony, who nonetheless tells them she will try to put things right. She promises to begin the legal procedures needed to exonerate Robbie, even though Paul Marshall will never be held responsible for his crime because of his marriage to Lola. As Briony leaves Cecilia's, she is optimistic about her role in Robbie's exoneration, thinking that it will be "a new draft, an atonement" and that she is ready to begin.
Postscript
The final section, titled "London 1999", is narrated by Briony herself in the form of a diary entry. Now 77, she is a successful novelist who has recently been diagnosed with vascular dementia, so she is facing rapid mental decline.
The reader learns that Briony is the author of the preceding sections of the novel. Briony attends a party in her honor at the Tallis family home, where the extended Tallis children perform The Trials of Arabella, the play that 13-year-old Briony had written and unsuccessfully attempted to stage with her cousins in the summer of 1935. Leon and Pierrot are in attendance, Jackson is fifteen years deceased, and Lola is alive but does not attend. Finally, Briony reveals to the reader that Robbie Turner died of septicaemia on the beaches of Dunkirk, that Cecilia was killed several months later when a bomb destroyed Balham Underground station during the Blitz, and that Briony's story of seeing them together in 1940 was a fabrication. Briony did attend Lola's wedding to Paul Marshall, but confesses she was too "cowardly" to visit the recently bereaved Cecilia to make amends. The novel, which she says is factually true apart from Robbie and Cecilia being reunited, is her lifelong attempt at "atonement" for what she did to them.
Briony justifies her invented happy ending by saying she does not see what purpose it would serve to give readers a "pitiless" story. She writes, "I like to think that it isn't weakness or evasion, but a final act of kindness, a stand against oblivion and despair, to let my lovers live and to unite them at the end."
