CAUTION: SPOILERS AHEAD
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, who is student body president and has a squeaky clean public reputation, promises Mrs. Caldwell that she will look out for the naïve Cecile. When Kathryn's stepbrother Sebastian Valmont enters the room, Mrs. Caldwell reacts to him coldly and leaves with Cecile.
Once they are alone, Kathryn displays her true colors - she tells Sebastian that she intends to use Cecile to take revenge on her ex-lover Court Reynolds, who dumped her for Cecile. Kathryn enlists Sebastian, a notorious womanizer, to seduce Cecile, thereby spoiling her in Court's eyes. Sebastian refuses, admitting that he is bored by the idea, and states that he is currently planning his greatest conquest to date: 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 car; if he succeeds, Kathryn will have sex with him.
Sebastian's first attempt to seduce Annette fails, as she is apparently already aware of his reputation. Sebastian initially suspects Greg, Annette’s friend from Kansas who is a popular football player and closeted homosexual, to be the rat who revealed his reputation to Annette. Sebastian convinces Greg to put in a good word about him to Annette, blackmailing him with a photo of him and Blaine, another gay student and Sebastian’s friend, in bed. After convincing Annette to tell him, Greg shares with Sebastian that Mrs. Caldwell was the one who warned Annette about Sebastian. Sebastian finally agrees to corrupt Cecile as revenge on Mrs. Caldwell. Meanwhile, Cecile confides in Kathryn about her romance with her older music teacher, Ronald Clifford. Kathryn reveals this to Mrs. Caldwell, who orders Cecile to end the relationship, but only because Ronald is Black. Sebastian lures Cecile to his house by claiming he has a letter to her from Ronald. He then gets Cecile drunk and blackmails 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 learn how 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 for resisting the chance for true love despite claiming that she was waiting for it. Annette finally relents, but Sebastian, confused about his own feelings, now refuses her. Annette flees. Sebastian finds her and professes his feelings, and they make love.
Kathryn offers herself to Sebastian after he wins the bet, but he rejects her, now only wanting Annette. Sebastian informs Kathryn that he is planning to tell Annette the truth, but a jealous and enraged Kathryn warns him that doing so will destroy both his and Annette's reputations. Sebastian lies to Annette, claiming that she was just a conquest 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, not Annette, was the true target of her scheme and that she manipulated him into abandoning Annette for her own amusement. She then dismisses him, telling him that she doesn't sleep with "losers".
Sebastian tries to contact Annette to confess the truth and beg for a second chance, 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, as well as his many conquests. Seeking revenge, Kathryn calls Ronald, claiming that Sebastian hit her and slept with Cecile. Ronald violently confronts Sebastian, and the ensuing fight migrates to the middle of the street. Annette tries to intervene but is thrown into traffic. Sebastian pushes her to safety but ends up getting hit by a car. Sebastian and Annette confess their love for each other before he dies.
On the first day of school, at Sebastian's memorial service, Kathryn delivers a eulogy, which people start leaving midway through. Kathryn searches the crowd, only to find Cecile handing out copies of Sebastian's journal. As the details of her manipulations and drug abuse are made public, Kathryn's reputation is finally destroyed.
(synopsis taken from Wikipedia)