Please note that in order to provide a useful analysis of this novel, it is necessary to reveal the novel's ending in the discussion below. It is England, 1935. Briony Tallis, 12 years old, decides to become a writer. Her first experiment in novelistic technique involves narrating from three different points of view an odd incident she witnesses from her bedroom window: her sister Cecilia undresses and steps into a fountain in the presence of Robbie Turner, the son of a family servant. Robbie has been educated at Cambridge under Mr. Tallis's patronage, and intends to become a physician. He and Cecilia are in love.

Briony's reconstruction of the incident is inaccurate, but she fails to recognize the lesson of her exercise in multiple perspectives: her version is sufficiently coherent for her to mistake it for reality. She jumps to further conclusions and causes Robbie's wrongful conviction and imprisonment for rape and Cecilia's permanent estrangement from her family.

The rest of the novel both elucidates and unravels the opening sequence. It is 1940 and Briony is becoming both a nurse and a novelist. Both roles represent her efforts to atone for her disastrous narrative misconstrual. As a nurse, she learns a new humility and cares for the appalling injuries of soldiers who, like Robbie, are suffering the war in France.

A more metaphysical atonement lies in her work as a novelist: we realize that we have been reading Briony's own rewriting of the initial events and her careful imaginative reconstruction of Robbie's experiences in the Dunkirk evacuation. She tells of her discovery of the actual rapist (if a rape it was), her decision to retract her accusations and her efforts to make amends with Robbie and Cecilia.

In a final section, set in 1999, the aging Briony, now a successful novelist, learns that she is developing progressive vascular dementia. Soon, her ability to remember and grasp reality will desert her. But she has finished writing her latest version of Robbie and Cecilia's story, the novel we have just read, and can rest.

Her atonement seems complete until we learn that Robbie died in France and Cecilia in the Blitz, and that the (relatively) happy ending we read was simply made up by Briony. Devastatingly, we learn that atonement for an error of fiction has been limited to fictional reparation. The lethal damage it has caused in the actual world is beyond mending . . . unless, of course, we accept the vertiginous truth that the damage described in this novel is itself also no more (or less) than a fiction.