In an upscale New York City mansion, wealthy and popular teenager Kathryn Merteuil discusses her prep school with Mrs. Caldwell and her daughter, Cecile. Kathryn promises Mrs. Caldwell that she will look out for the naive Cecile. When Kathryn’s stepbrother Sebastian Valmont enters the room, Mrs. Caldwell reacts to him coldly and leaves with Cecile.
Kathryn tells Sebastian that she intends to use Cecile to take revenge on her ex-lover Court Reynolds, who dumped her for Cecile. Kathryn asks Sebastian to seduce Cecile, thereby spoiling her in Court’s eyes. Sebastian refuses because he is planning to seduce Annette Hargrove, the headmaster’s virgin daughter, who has published an essay in support of chastity until marriage.
After some negotiation, they agree on a wager: if Sebastian fails to seduce Annette, Kathryn gets Sebastian’s vintage Jaguar XK140; if he succeeds, Kathryn will have sex with him, suggesting possibility of anal as well, since Kathryn is the only girl Sebastian has failed to bed.
Sebastian’s first attempt to seduce Annette fails, as she already knows about his bad reputation. Sebastian discovers that Mrs. Caldwell, Cecile’s mother, warned Annette about him. He finally agrees to corrupt Cecile out of revenge.
Meanwhile, Cecile confides in Kathryn about her romance with her same age music teacher, Ronald Clifford. Kathryn reveals the romance to Mrs. Caldwell, who orders Cecile to end the relationship, her disapproval arising out of race. Sebastian lures Cecile to his house, offering a letter from Ronald. He then gets Cecile drunk and blackmails her in order to trick her into allowing him to perform oral sex on her. The next day, Cecile confides in Kathryn, who advises her to be as promiscuous as possible to please Ronald.
Sebastian begins to truly fall in love with Annette, who returns his feelings but is still hesitant. Sebastian calls her a hypocrite because although she claims to be waiting for her one true love, she resists him when he chooses to love her back. Annette finally relents. But Sebastian, confused about his own feelings, now refuses her. Annette flees to the estate of her friend’s parents. Sebastian finds her and professes his love, and they consummate their relationship.
Kathryn offers herself to Sebastian after he wins the ‘bet’, but he rejects her: he now only wants Annette. Enraged and jealous, Kathryn insults his masculinity. Stung, Sebastian informs her that he was planning to tell Annette the truth. Kathryn warns him that doing so will destroy both his and Annette’s reputations. Sebastian lies to Annette, claiming he just wanted to see what she was like in bed, and that he has no real feelings for her. Devastated, Annette tells him to leave.
Sebastian informs Kathryn that he has broken up with Annette and now wants his reward for winning the bet. Kathryn reveals that he, and not Annette, was the true victim of her scheme: for her own amusement, she manipulated him into abandoning Annette once she realized that he truly loved her. She then dismisses him, telling him that she doesn’t sleep with “losers.”
Sebastian desperately tries to contact Annette to confess the truth and beg her forgiveness, but she refuses to see him. He gives her his journal, in which he has detailed Kathryn’s manipulative schemes, their bet, and his true feelings for Annette.
Ronald starts a fist fight with Sebastian after Kathryn tells him that Sebastian hit her and violated Cecile. Annette tries to intervene, but is thrown into traffic; Sebastian is hit by a car when he pushes her to safety. Before dying, Sebastian confesses his love for her and Annette tells him she loves him too. Ronald leaves the scene in shame, realizing Kathryn manipulated him.
In her eulogy at Sebastian’s funeral, Kathryn claims she tried to set an example for her late brother. When people start leaving midway through her speech, Kathryn rushes outside to find Cecile handing out copies of Sebastian’s journal. Kathryn’s spotless reputation is finally destroyed as the details of her manipulations and drug abuse are made public.
In the final scene, Annette drives away in Sebastian’s car with his journal at her side, remembering the good moments they shared together.