In the summer of 1935, 13-year-old Briony Tallis, an aspiring writer with a vivid imagination, witnesses a moment of sexual tension between her older sister, Cecilia, and Robbie Turner, the son of a family servant, which she does not fully comprehend. This misunderstanding is further complicated when Briony reads a passionate love letter intended for Cecilia that Robbie has mistakenly sent. Fuelled by confusion and her fanciful imagination, Briony interprets the situation based on her limited grasp of adult relationships. Later, when her cousin Lola is assaulted, Briony’s misconceptions about what she has seen between Cecilia and Robbie lead her to accuse Robbie of the crime, an act that changes the course of all three lives.

Robbie, the victim of Briony's false testimony, is forced to join the British Army in the early stages of World War II as an alternative to prison. As he endures the retreat to Dunkirk, the film captures the horrors and confusion of war. Separately, Cecilia has become a nurse, her life irrevocably altered by her separation from Robbie and the consequences of her sister's actions. Meanwhile, Briony, now a young nurse herself, begins to understand the gravity of her actions and strives for atonement. Despite her efforts, she must face the emotional and moral complexities of her pivotal decision and its ripple effects on everyone involved.