It is 1935, and Briony Tallis is a precocious, imaginative, and somewhat sheltered child of 13. When she misinterprets an interaction between her sister Cecilia and Robbie, the son of their charlady, she sets events in motion that will impact Cecilia, Robbie, and Briony herself for years to come.
Atonement is a novel that builds gradually, with McEwan's elegant prose immersing the reader in the world of the English upper class of the 1930's and then plunging into the chaos and horror of the British army's withdrawal to Dunkirk after the fall of France.
While all of the characters are strong and compelling, it is Briony who really carries the story, and in many ways, Atonement is a coming of age story for her. She begins the story as a child, largely shielded from the consequences of her actions. But when she chooses to act upon the adult stage, she must learn to live with those consequences, and to make her attempts at atonement.
Atonement was short-listed for the Man Booker Prize, and I really need to go and read the book that won that year, because if it was better than Atonement, it must be extraordinary. For another book set in the 1930's that features a single moment that has lasting consequences, try Amor Towles' Rules of Civility.