The plot of Atonement is centered around a romantic love story between Robbie and Cecilia. These two characters are separated by their socioeconomic status. Cecilia's family is wealthy, and Robbie and his family work on Cecilia's family's estate. Despite the apparent impossibility of their having a long-lasting relationship (like a marriage), the two seem to develop passionate feelings for one another.

This culminates in a scene in the library early in the novel, where Cecilia's younger sister Briony interrupts the two presumably having sex. Briony misinterprets this scene, thinking Robbie is hurting her sister, and this (in combination with a couple of other factors) leads Briony to accuse Robbie of raping her cousin Lola. The dramatic aftermath of this accusation leads Robbie and Cecilia to be separated forever, both dying during the impending world war.

One could argue that Briony's reconstruction of Robbie and Cecilia's story in her novel (apparently the book we have been reading) is an act of love. It's certainly meant as an act of "atonement"—to make up for how her actions separated the lovers forever. She admits, however, that she can never truly be forgiven because she is in control of the story and the wronged parties are dead. It is clear that Briony regrets her actions, though, and this could suggest a deep love for her sister.