"Atonement" is a book written in three major parts, with a final denouement from the author.
Part One tells the story of one day/night in 1935 at the Tallis family estate north of London, England. It focuses on Briony Tallis, the thirteen-year-old youngest daughter of three, who aspires to be a writer. She has written a play to be performed at dinner for the homecoming of her brother, Leon, and put on by herself and her three cousins who are staying with the Tallises for the summer because of a divorce between their parents. Before the play can be properly rehearsed, Briony witnesses a scene between her older sister Cecilia and the son of the family charwoman Robbie Turner. What is an innocent act is greatly misunderstood by the young imagination, and this sets off a series of events with eternal consequences.
Following the fountain scene, Briony intercepts a letter from Robbie to Cecilia and reads it. In it, she discovers perverse desires and sets out to protect her sister from this sex-craved maniac. Before she can do so, she witnesses the couple making love and mistakes it for assault, further confirming her assumption that Robbie is out to harm Cecilia.
Before the night is through, her twin cousins run away from home triggering the rest of the dinner guests to search for them in the dark night. Briony, who is searching alone, witnesses a rape taking place of her older cousin Lola. Not one to miss her opportunity, Briony convinces everyone at the scene, including authorities, that the assailant was Robbie Turner, and he is taken to jail.
Part Two takes place five years later. It follows Robbie Turner as he retreats through France as a soldier during the war. The reader has learned he served three years in prison for his crime and is now able to exonerate himself by serving in the army. Separated from his battalion, Robbie is marching through the countryside with two other corporals trying to get to the evacuation town of Dunkirk. During his march, Robbie experiences the atrocities of war, and has plenty of time to consider his situation as soldier, criminal, and victim of Briony's false accusations. The three men make it to Dunkirk which is in a state of complete chaos. Robbie is severely wounded but is determined to make it home to Cecilia who is waiting for him.
Part Three picks up the eighteen-year-old Briony who has signed up as a nurse in London. Suffering from guilt for her crime as girl, Briony hopes nursing will act as a penance for her sin. Briony is also still writing. She submits a story to a London journal which is rejected, but in the rejection she is encouraged to develop the story further as it is quite good. When the soldiers return from Dunkirk, Briony experiences the horrors of war first hand, and is humiliated at her failure to perform her duty. At the end of Part Three, Briony seeks out her older sister. Before she does, she attends the wedding of Paul Marshall (whom she knows to be Lola's rapist) and Lola. Briony does nothing to stop the marriage.
When she visits her sister, it is discovered that Robbie is still alive and living with Cecilia. This makes Briony happy to see. She does not so much as ask for forgiveness from the two lovers (who refuse it anyhow) as simply admit her guilt and seek counsel on what she can do to make it better. Robbie and Cecilia give Briony a list of instructions to follow that will help clear Robbie's name. Briony agrees to do each one, and heads back to work in London. The last we see of Robbie and Cecilia are on the tube station platform.
The final section of the boo, London, 1999, is a letter from the author to the reader. It is revealed here that the author is Briony herself. She explains that she was able to write the war parts of the book with the aid of letters form the museum of archives and a pen-pal relationship with one of the corporals with whom Robbie marched. Briony attends a birthday party/family reunion at her old home, the original scene of the crime. She also reveals that she is dying. In a final twist, Briony informs her reader that she has made up the part about visiting Cecilia and Robbie in London and how both people died in the war. Her act to let their love last forever in the pages of her book will be her final atonement to her crime.