In Westerburg High School in Sherwood, Ohio, Veronica Sawyer is part of a popular but feared clique that also consists of three other wealthy and beautiful girls with the same first name: Heather Chandler, Heather Duke, and Heather McNamara. However, she has grown tired of them and longs to return to her old life with her "nerdy" friends. Veronica becomes fascinated with Jason "J.D." Dean, a new student and rebellious outsider, after he fires blanks on Kurt Kelly and Ram Sweeney in response to them bullying him. She later attends a frat party with Chandler, but refuses to have sex with one of the members and vomits on Chandler. In retaliation, Chandler vows to destroy her reputation. J.D. arrives at Veronica's house, and the two have sex outside. They express to each other their mutual hatred of Chandler's tyranny.
The next morning, Veronica and J.D. break into Chandler's house, where J.D. puts drain cleaner in a mug for Chandler but Veronica dismisses his plan. She mixes orange juice and milk together instead, as an attempt to make her vomit and get revenge. However, J.D. hands Chandler the mug with the drain cleaner, killing her. Veronica panics but J.D. urges her to forge a dramatic suicide note in Chandler's handwriting. The school and community look on Chandler's apparent suicide as a tragic decision made by a troubled teenager, making her even more worshiped in death than in life. Duke soon steps into the role of the clique's leader and begins wearing a red scrunchie that had belonged to Chandler.
McNamara convinces Veronica to go with her, Kurt and Ram on a double date, during which the boys end up drunk and pass out in cow manure. The following day, they spread a false rumor about Veronica performing oral sex on them, ruining her reputation. J.D. proposes that she lure them into the woods and then shoot them with non-fatal bullets. J.D. shoots and kills Ram but Veronica misses Kurt, who runs away. J.D. chases Kurt back towards Veronica and kills him. J.D. plants material next to the boys implying that they were gay, and a note stating the two were lovers participating in a suicide pact. At their funeral, the boys are made into martyrs against homophobia. Although she keeps dating J.D., Veronica is increasingly disturbed by his behavior. Martha Dunnstock, a frequent target of bullying, pins a suicide note to her chest and walks into traffic. She survives but is badly injured and mocked by her peers. Later, McNamara calls a radio show to discuss her depression; the next day, Duke tells the entire school about the radio call. McNamara attempts suicide by overdosing in the girls' bathroom but she is saved by Veronica. She tells J.D. that she will not participate in any more killings and breaks up with him.
J.D. blackmails Duke into getting the students to sign a petition that unbeknownst to her is intended to act as a mass suicide note. Later, Veronica fakes her suicide to trick J.D., who assumes she is dead and reveals his plan to blow up the school during a pep rally. The next day, she confronts J.D. in the boiler room while he is planting dynamite. She shoots him and his switchblade cuts the wires to the detonator. J.D. follows Veronica outside with a bomb strapped to his chest, offers a personal eulogy, and detonates the bomb, killing himself. Veronica confronts Duke, takes the red scrunchie, and asserts herself as the new leader. She then invites Martha to spend prom night watching movies together, as Duke watches on.
Winona Ryder as Veronica Sawyer
Christian Slater as Jason "J.D." Dean
Shannen Doherty as Heather Duke
Lisanne Falk as Heather McNamara
Kim Walker as Heather Chandler
Penelope Milford as Pauline Fleming
Glenn Shadix as Father Ripper
Lance Fenton as Kurt Kelly
Patrick Labyorteaux as Ram Sweeney
Jeremy Applegate as Peter Dawson
Em Lodge as M.C. May
Renée Estevez as Betty Finn
Carrie Lynn as Martha "Dumptruck" Dunnstock
Chuck LaFont as Officer Milner
Phill Lewis as Dennis