Based on the best selling novel by Ian McEwen, Atonement is the story of 13-year-old Briony Tallis (Saoirse Ronan), an aspiring writer from an upper class English family who because of a mix of immaturity, misunderstanding, and jealousy, tells a lie that changes her life and that of her family forever.

Briony's older sister Cecilia (Kiera Knightley) is currently attending Cambridge University, alongside Robbie Turner (James McAvoy), the son of the Tallis family housekeeper (Brenda Blethyn). Cecilia's father, for some unknown reason, has been paying Robbie's way through school.

As the film opens in England, 1935, Robbie is spending the summer working in the garden at the Tallis estate while waiting to start medical school. Meanwhile, Briony has just finished her first play titled The Trials of Arabella, which she describes as about "the complications of love". Briony intends to have her visiting cousins, fifteen-year-old Lola Quincey (Juno Temple) and her younger twin brothers (Felix and Charlie von Simson) help her perform as that evening's entertainment. Unfortunately for Briony, all the kids want to do on this sweltering summer day is enjoy a swim, rather than rehearse her play, leaving the bossy, precocious young girl alone in her bedroom.

From her bedroom window, Briony sees Cecilia out on the lawn by the water fountain talking to Robbie Turner. While Briony is trying to draw her own conclusions as to what the moment means, Cecilia and Robbie are clearly fighting strong feelings for one another. Shortly thereafter, the Tallis sisters' older brother Leon (Patrick Kennedy) arrives with a friend, chocolate magnate Paul Marshall (Benedict Cumberbatch). Much to Cecilia's dismay, Leon has invited Robbie to dine with the family that evening.

Director Joe Wright (Pride and Prejudice) fills the first act of Atonement with such beauty — the lushness of the English countryside and the clipped perfection of Kiera Knightley's upper class British accent — it's easy to believe this film will be a proper love story. However, as anyone who has read McEwen's novel knows, that is not meant to be. Wright directs the early scenes involving Briony, Cecilia, and Robbie brimming with apprehension and nervous energy. Robbie does something sophomoric while composing a note to Cecilia, yet that brief, unguarded moment will irrevocably change their lives forever.

The film then jumps a few years ahead to the start of World War II. Gone are the lush green lawns of the Tallis estate. Robbie has now enlisted in the army and been posted to France. Cecilia, a nurse in London, has never forgiven Briony for her false accusations years earlier. Briony (now played by Romola Garai), now eighteen and training to be a nurse herself, aches to undo the terrible wrongs she committed. There is one meeting between Cecilia, Robbie, and Briony in London that drives home the point of just how much each of them has lost.