In 1935, 13-year-old fledgling writer Briony Tallis (Saoirse Ronan) and her family live a life of wealth and privilege in their enormous mansion. On the warmest day of the year, the country estate takes on an unsettling hothouse atmosphere, stoking Briony's vivid imagination. Robbie Turner (James McAvoy), the educated son of the family's housekeeper, carries a torch for Briony's headstrong older sister Cecilia (Kiera Knightley). Cecilia, he hopes, has comparable feelings; all it will take is one spark for this relationship to combust. When it does, Briony - who has a crush on Robbie - is compelled to interfere, going so far as accusing Robbie of a crime he did not commit. Cecilia and Robbie declare their love for each other, but he is arrested - and with Briony bearing false witness, the course of three lives is changed forever. Briony continues to seek forgiveness for her childhood misdeed. Through a terrible and courageous act of imagination, she finds the path to her uncertain atonement, and to an understanding of the power of enduring love.