Briony Tallis is a literary, self-important 13-year-old who lives in an English country estate in 1935.
Her cousins, 15-year-old Lola Quincey and 9-year-old twins Jackson and Pierrot Quincey, are coming to stay with the Tallises because their parents are embroiled in a divorce. Meanwhile, Briony’s older sister Cecilia holds unresolved romantic feelings for Robbie Turner, the Tallises’ gardener (Robbie’s romantic feelings for Cecilia, meanwhile, are passionately resolved). Thanks to the Tallises’ funding, Robbie studies with Cecilia at Cambridge and plans to become a doctor.
From a window of the estate, Briony witnesses the two of them accidentally break a family heirloom vase in front of a fountain. When Cecilia removes her clothes in front of Robbie to retrieve the shards from the fountain, Briony starts to think Robbie is a threat to her sister.
Later, Robbie gives Briony a letter of apology to give to Cecilia, but accidentally hands her a vulgar draft instead. Briony reads the letter and becomes convinced Robbie is a menace. When Robbie realizes his error, he goes to Cecilia to apologize. This apology turns to passionate lovemaking in the family library. Briony enters the room and misinterprets the situation