McEwan's large, class conscious drama investigates the impact of a child's imagination. Young Briony Tallis is fanciful; in her fancy, she creates worlds and characters that are as real to her as the distant adults that haunt the large beautiful house in which she lives. Briony is a child of the mind, and she has no understanding of empathy. To Briony, there is no other truth than the truth her imagination will allow her understand. When she witnesses a scene between her older sister, Cecilia and the housekeeper's son, Robbie, she sees anger, humiliation, and power. She does not understand that these three forces are firmly on Cecilia's side; rather she views Cecilia's near naked jump into the fountain as an act of submission to a supposed vile, sex-obsessed Robbie. Later, when she reads a letter she should not have seen, her ideas are fixed. A few more seemingly connected incidents prove her suspicions, and her innocence and desire to protect her sister turn into a rampaging evil.
Briony's mistake is understandable for her age and reading practices, (though the adults' blind belief of her observations are not), but it is too dire to be forgiven, and Cecilia and Robbie's lives are irrevocably changed. Briony's efforts to atone for her wrong ultimately takes the novel into the scope of analyzing the role of fiction. Briony grows up to become a writer, and the words and acts that are not realized in her life are woven into her fiction. The novel ends with a dramatic point, one that Briony intuitively knew when she was 13, that fiction, whether written or imagined, can change the world.
For a more in-depth analysis of this novel, see James Woods' review here.