This whole story is based on a misunderstanding that goes horribly wrong. Cecilia, a young woman is home for the summer from school. Robbie, the gardener's son is also at home. Cecilia decides to put some flowers in a very expensive vase to welcome the arrival of her brother and his friend. She sees Robbie in the garden and he volunteers to do this for Cecilia, however she wants to do it herself, as they fight to hold the vase, the handle breaks off and it falls into a fountain. Cecilia must retrieve the handle so she strips down to her underwear to dive into the fountain to retrieve it. As she emerges from the fountain, Briony her younger sister sees this from the window and sees Robbie watching her soaking wet sister in her underwear. Meanwhile, the brother and his friend arrive and he invites Robbie to dinner. Robbie decides to attend, and bring a note for Cecilia as he has fallen for her. He makes up many different drafts including a very vile and sexual note as a joke. He finally decides on a note and as he is getting dressed, he quickly picks up the note and heads out. He sees Briony playing outside on the outskirts of the property and he decides its better for Cecilia to see the note before he arrives so he gives the note to Briony to give to Cecilia. Shortly after, he realizes he gave the wrong note to Briony. Briony out of curiosity reads it before actually giving it to Cecilia. When Robbie arrives, he and Cecilia go into the library and he tries to explain to Cecilia. They both realize they are in love and they make love in the library. At the same time, Briony is looking for Cecilia and sees Robbie and her in the library in way such that Robbie looks like he is in an attacking position. During that night, the friend that was visiting the family with the brother rapes the family's cousin who is living with them. The police come and Briony tells them all she has seen and misunderstood. The cousin that was raped did not see who it was because it happened in the dark outside. The police end up arresting Robbie. Robbie must go to war. Cecilia is enraged, cannot stay with her family any longer decides to become a nurse. Briony does as well. Years later, Briony realizes that the friend is the one who raped her cousin, especially after she learns they are getting married. Briony continually tries to atone for her actions. Alas, Robbie dies at war, Cecilia also dies in a flood. All because of a misunderstanding.