Part oneIn the hot summer of 1935, thirteen-year-old Briony Tallis is already an ambitious writer. She has written a play for her older brother, Leon, who is supposed to arrive later in the day. The characters are to be played by her cousins, Lola, fifteen, and the twins Jackson and Pierrot, nine. Briony's sister, Cecilia Tallis, has returned home from Girton College, Cambridge. She is trying to sort out her confused feelings towards the charlady's son and her childhood friend, Robbie Turner, who is also home from Cambridge (Cambridge University) for the summer. His studies were financed by her father, Jack Tallis.
Cecilia wants to fill a vase with water at the fountain in front of the Tallis' house. She meets Robbie and they start talking, but the conversation quickly becomes awkward. Both are irritated. When Robbie wants to help Cecilia with the vase, she remains stubborn, the vase breaks and two pieces fall into the fountain. Cecilia strips to her underwear, jumps into the fountain and retrieves the fragments while Robbie only stares at her. Briony witnesses the ensuing moment of sexual tension from an upstairs bedroom and is confused as to its meaning.
Leon Tallis arrives with his friend, Paul Marshall. They meet Robbie on their way to the house, and Leon invites him to dinner. Cecilia is irritated at Robbie's coming, but does not know why he bothers her so much.
Meanwhile, Robbie wants to write a letter to Cecilia to apologize for his behaviour at the fountain. He indicates that he also feels awkward around her, and, like her, does not know why. After finishing it, he unthinkingly adds an obscene suggestion on to the bottom of his letter, using the word "cunt". Although he then writes another version of it, it is the first that is accidentally delivered to Cecilia via Briony, who reads it. Briony consults with her cousin Lola; Briony is then convinced that Robbie is a "maniac" and that she must protect her sister Cecilia from him.
Upon reading Robbie's letter, Cecilia realises her love for Robbie, and they end up making love in the library. Briony interrupts them, and interprets their lovemaking as a sexual assault upon her sister. During dinner, the twin cousins run away, leaving a letter. The dinner party divides into groups to go out searching for them. Robbie and Briony are the only ones who leave alone - as Robbie has to acknowledge later, the biggest mistake of his life. In the dark, Briony comes across Lola being raped by an unknown attacker. Briony convinces herself that she saw Robbie - it fits perfectly in her picture of him as a maniac. Lola, afraid of even more humiliation stays quiet and lets Briony do the talking.
The police arrive to investigate, and when Robbie arrives with the rescued twins, he is arrested solely on the basis of Briony's testimony. Apart from Robbie's mother, only Cecilia believes in his innocence.
Part twoThe Second World War has started and Robbie has spent three years in prison. He is released on the condition of enlistment in the army. Cecilia has become a nurse and has cut off all contact with her family. Robbie and Cecilia have only been in contact by letter, since she was not allowed to visit him in prison. Before Robbie has to go to war in France, they meet once for half an hour during Cecilia's lunch break. Their reunion starts in awkwardness but they share a kiss before leaving each other. In France, the war is going badly and the army is retreating to Dunkirk (Dunkirk evacuation). The injured Robbie is on his journey to the evacuation place, thinking about Cecilia and past events like teaching Briony how to swim and reflecting on Briony's possible reasons for accusing him. His single meeting with Cecilia, the one kiss is the memory that keeps him walking, his only aim seeing her again. At the end, Robbie falls asleep in Dunkirk (Dunkirk), one day before the evacuation, his fate unknown.
Part threeBriony has refused her place at Cambridge, and instead is a trainee nurse in London. She has realized the full extent of her crime - stating a false testimony and more importantly the impact it had - and now remembers it was Paul Marshall, Leon's friend, who she saw raping Lola. It gives the impression that Briony sees her work at the hospital as an atonement. At the same time Briony still is a writer, although she does not pursue it with the same recklessness as she did as a child.
In a crucial scene, Briony is called to the bedside of Luc, a young French soldier who is fatally wounded. She consoles him in his last moments by speaking with him in her school French, and he mistakes her for an English girl whom his mother wanted him to marry. Just before his death Luc asks "Do you love me?" to which Briony answers "Yes"-not only because "no other answer was possible" but also because "for the moment, she did. He was a lovely boy far away from his family and about to die". Afterwards, Briony daydreams about the life she might have had if she had in truth married Luc and gone to live with him and his family. (Later it is briefly mentioned that after the war Briony married a Frenchman named Thierry in Marseille).
Briony attends the wedding of her cousin Lola and Paul Marshall before finally visiting Cecilia. Robbie is on leave from the army and Briony meets him unexpectedly at her sister's place, as well. They are still furious and unforgiving at guilt-ridden Briony who tells them she will try and put things right. She promises them 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, his victim.
Part fourThe fourth section, titled "London 1999", is written from the perspective of Briony. She is a successful novelist at the age of seventy-seven and dying of vascular dementia.
It is revealed that Briony is the author of the preceding sections of the novel. Although Cecilia and Robbie are reunited in Briony's novel, they were never reunited in reality: Robbie Turner died of septicemia caused by his injury on the beaches of Dunkirk, and Cecilia was killed by the bomb that destroyed the gas and water mains above Balham Underground station. The truth is that Cecilia and Robbie never saw each other again after their half-hour meeting. Though the detail concerning Lola's marriage to Paul Marshall is true, Briony never visited Cecilia to make amends. She was too much of a coward to face her sister shortly after Robbie's death. Briony explains why she decided to change real events and unite Cecilia and Robbie in her novel, although it was not her intention in her many previous drafts. She did not see what purpose it would serve if she told the readers the pitiless truth: they could not draw any sense of hope or satisfaction from it. But above all, she wanted to give Robbie and Cecilia their happiness by being together. Since they could not have the time together they so much longed for in reality, Briony wanted to give it to them at least in her novel.
The novel ends with a meditation on the nature of atonement and authorship. The conclusion that Briony reaches is that an author cannot achieve atonement through a novel: the author plays the role of God in his novel, he determines his characters' fates and therefore can alter them as he likes.
In the actual novel written by Ian McEwan, the reader knows that in fact Cecilia and Robbie died and were never reunited, and that Briony was haunted all her life by having irreversibly deprived them of their chance for happiness. However, Briony chooses to end her novel (written within McEwan's) with part 3, wherein the two lovers survive the war and embark on a happy life together.