In the summer of 1935, Briony (Soriz Ronan), a 13-year-old girl from a well-off British family, had just started to try writing and had a rich imagination. One day, she secretly discovered that the servant's son, Robbie Turner (James McVoy), had an ambiguous relationship with her sister Cecilia (Kate Knightley), and found that he had written her a love letter full of erotic meanings. Briony's rich imagination made up all kinds of terrible things and imagined them. When her cousin Lola was raped, Briony was quite sure that she saw Robbie did it. This accusation caused Robbie to be wrongly imprisoned, leaving Cecilia heartbroken. World War II broke out. After being released from prison, Robbie and Cecilia both joined the battle to defend their motherland. Briony gave up going to Cambridge to study and became a military medical staff. No one could understand her behavior. Only she knew that she was carrying a huge secret and self-blame. She mustered up the courage to solve this secret so that she could truly atone for her sins and be forgiven. However, things change beyond Briony's expectations.