NOTE: This Spoiler is sent in by D Schwarz.
The film starts out with a 13 year old Briony Tallis (Saoirse Ronan) writing a play that she wants to put on that evening for the return of her older brother Leon. She runs through the house and into the garden and finds twenty-something year old Robbie Turner the housekeeper's son (James McAvoy) as he is tending to the flowers. They exchange conversation and its clear that she has a crush on him but he regards her as a little sister. We then see Briony's mother reading the play and loving it.
The next scene shows Briony lying on the garden with her older sister Cecilia (Keira Knightley). Briony asks Cecilia why she doesn't talk to Robbie anymore, Cecilia says she does, they just move in different circles. Then we see Briony rehearsing the play with her cousins, Pierrot and Jackson Quincy and their older sister Lola (Juno Temple). Lola is about 16 or so. The younger boys quickly get bored with the play and go off for a swim and Briony if left alone in the house. She is looking out the window and sees Cecilia and Robbie out by the fountain in some sort of argument. Then we see Cecilia strip down to her slip and jump into the fountain to retrieve something and she comes out and her slip is wet and sheer and Robbie looks away as she dresses and leaves. We suddenly see the same scene from the point of view of Cecilia and Robbie, as they are talking while walking into the garden, Cecilia is holding an expensive vase and as she tried to fill it with water from the fountain, Robbie tried to take it from her to fill it and she refuses and they pull on it, breaking the porcelain. It falls into the fountain, which is what Cecilia goes to retrieve.
We then see Cecilia in her room, smoking, just generally getting ready for the evening. Through her window she sees her brother Leon drive up with a friend, some rich tycoon who owns a chocolate factory. She sees Leon talking to Robbie with dismay. They meet Leon's friend, who seems rather shady. Leon, Cecilia and the friend, Paul Marshall, are all swimming I the lake and it is revealed that Robbie and Cecilia went to school together and that Robbie is pursuing a medical degree. Leon informs Cecilia that he has invited Robbie to dinner, and Cecilia is upset at this. The scene then cuts to Robbie in his cottage typing a letter to Cecilia, trying to apologize for his behavior earlier that day. He types several drafts before finally typing a rather raunchy sexual letter to her. While he is typing we see the scene cut back and forth between him and Cecilia getting ready and putting clips in her hair. (Important) He laughs as he writes it, then folds it up and places it next to his typewriter. He then handwrites a very heartfelt apology and goes to get ready for dinner that evening. As he walks down the lane to the main house he sees Briony playing in the garden and asks her to deliver the letter to Cecilia, which she does. As she is running to the house, we see Robbie suddenly remember that he put the wrong letter in the envelope and in fact, the raunchy letter is being delivered. He screams for Briony but its too late and she is already in the house and is reading the letter. She then delivers it to Cecilia, but without the envelope, so Cecilia knows that she has read it. We then see Paul Marshall wandering through the house and comes across the cousins, Pierrot, Jackson and Lola. He entertains the children but it is also clear that he is interested in Lola.
Later on Briony goes to her room to change for dinner and Lola enters her room, complaining about being tortured by her little brothers. Briony takes this opportunity to tell Lola what was written in the raunchy letter, calling Robbie a sexual maniac. She then gets ready for dinner and is running down the stairs when she sees a small hairclip on the floor. She picks it up and sees a light shining through a doorway, she follows it to see Robbie and Cecilia in an awkward position against the wall of the library, obviously having made love just moments before. We see Briony's teary face just as the scene cuts to Robbie ringing the doorbell for dinner. Cecilia answers the door and he tried to apologize for giving the wrong version of the letter. Cecilia takes him to the library and basically they admit their attraction to one another and start kissing and having sex up against the wall. In the middle of it they tell each other they love one another and then continue, until Cecilia hears the door open and Briony walk in. They disentangle themselves and leave the room, with Briony just watching them.
At dinner they all pretend nothing's happened, until Mrs. Tallis notices that the young twin boys aren't at dinner so Briony is sent to get them. She comes back down and informs the group that the boys have run away, so a search party is immediately formed. Briony takes a flashlight and set out alone and stumbles across a man with his pants down on top of Lola. As soon as the flashlight shines on them he runs away, but you don't see his face. Lola is crying and Briony runs up to her, asking her who the man was. Lola isn't sure as her eyes were covered, but Briony convinces her its Robbie.
Lola is brought into the house and the police are called and Briony testifies that it was Robbie who attacked Lola and she is sure she saw him. Cecilia tells the police not to believe Briony as she is a fanciful girl but as soon as Robbie comes back to the house, having found the two young boys, the police take him away.
The movie cuts to 4 years later and we see Robbie in France with two other privates in the army, as it is now WW2. They ask him what an educated man like him is doing as a private, to which Robbie responds, you can't get promoted if you've come out of prison and he was given a choice, stay in prison, or join the army. Then we see a scene from six months earlier where Robbie and Cecilia meet after 3 and 1/2 years and after some awkward conversation they declare their love for one another after all this time. Cecilia has become a nurse now in London and is living in an apartment and doesn't speak to her sister Briony or her family anymore. Robbie returns to the war but they continue to write letters, Cecilia tells him of a cottage by the sea they can visit the next time he is on leave and gives him a picture of it to hold on to. We see him throughout the film, going through battle and looking at the same picture every chance he gets. Cecilia also tells him that Briony is now in Nurses training and has written to her, asking to talk.
We then see a 18 year old Briony, in her nurses uniform, trying to fit in at the hospital. She writes to Cecilia, asking to talk and telling her she only now realizes the full extent of her false testimony years ago. We see Robbie wounded in France and waiting for the roundup of soldiers to be taken to the hospital and then see Briony and the other nurses tending to injured soldiers. She imagines that she sees Robbie but he isn't there. Several days later Briony then leaves the hospital to attend the wedding of Paul Marshall, who is marrying her cousin Lola. Lola sees Briony at the wedding but ignores her. Briony then goes to Cecilia's apartment and begs to talk to her, to apologize. She sees Robbie there, who wants to break her neck but instead Robbie and Cecilia make Briony fix what she has done wrong and change her statement. Briony admits that she saw Paul Marshall attacking Lola that night in the garden and they become upset because Paul has just married Lola so he has immunity. We see Briony agree to change her statement and she leaves and we see Cecilia and Robbie finally together in the apartment, kissing. Then Briony is on a train and suddenly the screen goes black and a woman says, can we stop for a moment.
We cut to present day and we see an old Briony Tallis (Vanessa Redgrave) being interviewed about her latest book, titled Atonement. She explains how it is her last book because she is dying and she wanted to be completely honest about all the events of the situation. She admits however that she wasn't honest about one part. That she never went to her sister that day to ask for forgiveness and that Robbie was never there because he died of septicemia in France waiting to be taken to the hospital a few months earlier. She also says that she never got to set things right with her sister because she was killed in a bomb blast a few months after Robbie died. Briony admits that she has had to live with this truth all her life so she changed it in her book, in order to give Cecilia and Robbie their chance to be together in her book, if not in real life.
The last scene shows Cecilia and Robbie playing on the beach and then going into the cottage that Cecilia had talked about earlier.