On a summer day in the English countryside in 1935, thirteen-year-old Briony Tallis has written a play, The Trials of Arabella, to be performed by her visiting cousins, fifteen-year-old Lola and Lola’s nine-year-old twin brothers. The cousins are staying with Briony’s family while their parents finalize their divorce. Briony hopes the play will impress her brother Leon, who is home visiting with his friend, Paul Marshall. Briony is first dismayed by how immature she feels in comparison to Lola and then horrified when her cousins’ delivery of her lines does not match what she had imagined. Briony decides that she is done writing plays and that she is ready to move from childhood to adulthood. Meanwhile, Briony’s sister Cecilia has recently returned home from Cambridge. She has graduated, along with the Tallises’ charlady’s son and Cecilia’s childhood friend Robbie, and she is not sure what to do next. After breaking a vase while attempting to fill it with water from a fountain, Cecilia grows frustrated with Robbie’s attempts to help her, so Cecilia undresses, dives into the fountain, and retrieves the vase herself. Briony watches this interaction from inside the house, confused about what she has seen.

Robbie, who has been invited to dine with the Tallises that night, returns home and realizes he has fallen in love with Cecilia. He begins typing a letter apologizing for his awkwardness and includes a few lines graphically describing how he would like to make love to Cecilia. Knowing he cannot send that version of the letter, he writes another draft out by hand, which he plans to deliver. On his way to the Tallis house, Robbie asks Briony to deliver the letter to Cecilia. However, once Briony is gone, Robbie realizes he gave her the typewritten version instead of the handwritten one. Briony reads the letter before delivering it to Cecilia, disgusted by what it says. Briony confides in Lola about what Robbie’s note says, and they both agree that he is a maniac. Cecilia greets Robbie after reading the letter, and they go into the library to talk. Cecilia tells Robbie she has realized that she has had feelings for him for some time, and the two begin to make love until Briony walks in on them. Cecilia and Robbie are frustrated by being interrupted, but Briony believes she has just saved her sister from an assault.

During dinner, the twin boys ask to be excused. After they leave, Briony notices a note they left behind saying they have run away. The group breaks up into search parties to look for them on the grounds. Briony goes off by herself and eventually comes across Lola being raped by a man who runs away before Briony can see him. Lola claims that she did not know who the attacker was. Due to everything she has witnessed that day, Briony feels certain that the man was Robbie. Back at the Tallis house, she gives statements to the police saying that she saw Robbie assaulting Lola. Robbie returns at dawn, having found the twins, and is arrested. Cecilia becomes distraught and promises Robbie she will wait for him.

After being convicted, Robbie spends three years in jail before being released early in exchange for joining the armed forces. Having written to each other during Robbie’s time in jail, he and Cecilia are able to meet once in a café before war breaks out. They continue to write to each other while Robbie is fighting in France. Cecilia has begun working as a nurse and has cut off her family for their role in Robbie’s conviction. In one letter, Cecilia tells Robbie that Briony is also training to be a nurse and has written explaining that she has realized she was wrong about Robbie and would like to revise her testimony. While making his way to Dunkirk, where the British forces are to evacuate, Robbie comforts himself with the thought that his conviction for the crime he did not commit may be overturned. While traveling with two corporals, Robbie sees many horrors of the war and helps people when he can. He eventually makes it to Dunkirk and, upon learning there will be boats to take the soldiers to Britain in the morning, falls asleep in a bombed-out house.

Briony, now eighteen years old, has chosen nursing over Cambridge as punishment for falsely accusing Robbie, although she believed her testimony to be true at the time. On learning that Lola and Paul Marshall are to be wed, Briony’s suspicion that Paul was the actual rapist is confirmed. After the evacuation from Dunkirk, Briony tends to several wounded and dying soldiers, seeing Robbie in each of them and hoping that he is not dead. On the day of Lola and Paul’s wedding, Briony goes to the church and considers objecting to their marriage but stays silent. After, she goes to Cecilia’s flat, where she finds her with Robbie. Although Cecilia and Robbie have clearly not forgiven her, Briony is relieved that Robbie is alive and the two of them are together. Briony promises them she will revise her testimony and tell their family the truth about Lola’s assaulter.

In 1999, Briony is turning seventy-seven years old and dying of vascular dementia. She goes to the Imperial War Museum to attend to a few remaining matters, where she sees Lola and Paul Marshall, who have become wealthy philanthropists. Noting that Lola is in excellent shape, Briony understands that her latest novel will not be published until after Briony is dead, as she cannot publish it while Lola and Paul are still alive. Her novel is the story that has been presented until this point in the book and is as true to life as it can be, except for the ending, as Cecilia and Robbie never got the chance to reunite. Robbie died before the evacuation at Dunkirk, and Cecilia died a few months later in a London bomb raid. Briony has spent her life trying to make up for what she did to them, and this novel is her way of giving them the happy ending they never had. That night, Briony gathers with her family for her birthday celebration, and the children perform The Trials of Arabella. Briony pictures Cecilia and Robbie gathered with them, together and happy, before going to bed.