Known for telling stories about problematic teens and dysfunctional family relationships, Ian McEwan does not stray too far from his roots in Atonement. The first half of the novel focuses on one day in a seemingly idyllic setting, when suddenly everything goes wrong. At the core of the whirlwind circumstances that change the lives of the people around her is a thirteen-year-old girl with a significant imagination and maybe just a touch of cruelty in her eyes.
When she was ten, Briony Tallis confessed her pre-teen infatuation for Robbie Turner, the twenty-year-old son of the Tallis family's housekeeper. Three years later, when the novel opens, Briony witnesses Robbie and Briony's older sister, Cecilia, flirting with one another. Briony, through interior monologue, does not understand what is going on between Robbie and Cecilia. She is disturbed when she reads a note Robbie has sent to Cecilia that has obvious sexual overtones. When she catches Robbie and Cecilia in a dark corner of the family's library in the throes of passionate sex, Briony fears for her sister's life. Or at least, that is what she tells the reader. That might even be what she tells herself. However, this does not fully explain what she does next.
At the end of celebratory dinner, which includes Briony's family, three visiting cousins, and a wealthy friend, Briony's nine-year-old twin cousins excuse themselves from the table, leaving behind a note that states they are running away. In the dark of a hot summer night, the family goes out onto the grounds of the family estate and search for the missing boys. While all the other family members are calling out the boys' names, Briony is by herself, scheming.
Briony happens upon her sixteen-year-old cousin Lola, who is crying. As Briony approaches, a male figure recedes into further darkness. Lola has been raped. Briony, still under the influence of her fear and disappointment, identifies the rapist as Robbie. And so the disintegration of the family begins. Robbie is sentenced to jail. Cecilia cuts ties with her family. Lola and her rapist hide behind the lie. And Briony is left to atone for her sin.