Sarah Bailey (Robin Tunney), a troubled teenage girl who has previously attempted suicide, has just moved from San Francisco to Los Angeles with her father (Cliff De Young) and stepmother. She enrolls in a local Catholic high school, but has trouble fitting in. During French class, her classmate Bonnie (Neve Campbell) witnesses Sarah telekinetically causing a pencil to rotate while standing on its tip.
During lunch, Sarah is hit on by Chris (Skeet Ulrich), the school's football star. She asks about Bonnie and her two friends Nancy (Fairuza Balk) and Rochelle (Rachel True). Chris tells Sarah to stay away from the trio, because "they're witches". Bonnie tells Nancy and Rochelle that Sarah is the "fourth" who will complete their circle and make a full coven. The three girls each have issues: Nancy lives in a trailer with her mom, Grace Downs (Helen Shaver) and abusive stepfather Ray (John Kapelos) , Bonnie has massive burn scars all over her back, and the painful treatment recommended by the surgeon (Brenda Strong) is likely to fail. Insecure African American athlete Rochelle is subjected to racist taunts by the most popular girl in school, blonde Laura Lizzie (Christine Taylor).
After school, the three girls befriend Sarah and take her to an occult shop. The owner, Lirio (Assumpta Serna), comments that Sarah is not like the other girls and says to her: "Maybe you are a natural witch; your power comes from within." While leaving the shop, Sarah is harassed by a vagrant (Arthur Senzy), and all four girls simultaneously will for something to happen; the vagrant is then hit by a car. The girls escape, and Nancy is thrilled at their "connection". Nancy tells Sarah about "invoking the Spirit" Manon, which is their ultimate goal as a coven.
Sarah leaves the girls to meet Chris, but refuses to have sex with him. However, at school the next day, Sarah discovers that Chris boasted to the whole school that they slept together, and that she was the worst he's ever had. Nancy, Bonnie and Rochelle comfort Sarah, and invite her on a field trip. They take the bus out to the country. The bust drivers tells them to be careful about wicked boys, but they say that they four are the dangerous ones. While on the countryside, they call the corners and cast some spells: Rochelle asks for the strength not to hate those who hate her, Sarah performs a love spell on Chris, Bonnie asks for beauty inside and out, and Nancy asks for "all the power of Manon."
Shortly after the spells are cast, they show signs of working: Chris becomes infatuated with Sarah and goes after her constantly in spite of his friends Mitt (Breckin Meyer) and Trey (Nathaniel Marston) taunting him about it, Bonnie's scars miraculously heal, and the next time Laura bullies Rochelle, Laura's hair begins falling out. At home, Nancy causes the microwave and all light bulbs to explode, and Ray suffers a massive heart attack and dies. Then Nancy and her mother are told by the insurance man (Brogan Roche) that they have inherited $175,000 from an insurance policy, and they feel overjoyed about it. Nancy and her mother move into a posh high-rise where the girls meet one night and learn disguise-changing magic. Nancy's mother looks confused and dazzled. She has bought a weird-coloured sofa and a jukebox with only songs by Connie Francis. The four friends lock themselves in Nancy's room ignoring Nancy's mother, who is left empty and ignored, just as she felt before cashing in on the insurance money.
The girls go to Lirio's shop again, where Nancy finds a book about "Invoking the Spirit". Later that night, the girls go to the beach and form a circle, calling on Manon. At the culmination of the spell Nancy is struck by lightning. The next day the rest of the girls witness Nancy walking on the water, and from this point on, Nancy's powers have increased.
Later on, Rochelle sees a balding Laura in the locker room, hysterically sobbing after swim practice. Laura's two closest friends (Elizabeth Guber and Jennifer Greenhut) try to offer some consolation, to no avail. Rochelle feels remorse for the spell she has cast and when she looks in the mirror at herself, her reflection looks away.
Sarah's love spell also backfires on her. She finally accepts to have a dinner date with Chris, but he takes her to the top of a hill and attempts to rape her. Sarah runs away and knocks on Nancy's door. Nancy leaves her home for a party Chris is attending in order to punish him. However, when she arrives to the party, she tries to seduce Chris by disguising herself as Sarah, but when the real Sarah arrives as they engage in foreplay, Nancy causes Chris to fall out a third-story window, killing him.
On consequence of this, Sarah starts having nightmares concerning her old friends, and feels like they're following her everywhere to torment her. She tries to stop Nancy - who has become the leader of the three remaining coven witches - by binding her powers so that she won't be able to hurt other or herself, but this does not work and the three other coven girls now hate Sarah. Nancy appears and tells Sarah that in the old days, if a witch betrayed her coven, they would kill her. Needing help, Sarah goes to Lirio, who tells her to invoke the spirit herself. Lirio also reveals that Sarah's mother was a powerful witch, and that her talent has passed on to Sarah. Sarah starts to invoke Manon, but she has a vision of fire and the shop exploding, so she leaves the shop absolutely terrified..
Sarah returns home, where she is tormented by Nancy, Bonnie and Rochelle, who taunt her using magic and intimidate her with threats on her life. Sarah arrives home and nobody is there. She is instructed to turn on the TV set and listens that her family went back to San Francisco thinking that she had run away there, and that their plane had had an accident without any survivors. Without a second of rest, all kinds of snakes and worm appear everywhere. The three other coven girls then appear. After a few seconds of intimidation Nancy says that Sarah will commit suicide.
Nancy later slashes Sarah's wrists and a letter on Sarah's handwriting stating the reasons of her suicide magically appear - blaming herself for Chris' death. Sarah runs to her room, while Nancy, Bonnie and Rochelle wait for her to die. Rochelle then tells Nancy that this has gone out of hand. Nancy tells her to go upstairs to check on Sarah and threatens to slit her throat if she doesn't. Bonnie pulls up Rochelle to Sarah's room. Sarah is almost too weak to invoke Manon, but then she hears her mother (Janet Eilber) moving in an old photograph and her sweet voice whispering, "Don't be afraid. Reach inside yourself". Sarah successfully invokes Manon and is able to cast spells to fight back, as well as heal the injuries to her wrists. Forced by a counter spell of Sarah's to see themselves and what they've become by catching their reflections in a mirror, Bonnie's face horribly scarred and Rochelle's hair falling out, Bonnie and Rochelle flee, leaving only Nancy and Sarah at home.
Nancy and Sarah have a showdown, in which Nancy is ultimately defeated by Sarah as she binds Nancy's power to prevent her from doing any more harm.
In the end, Nancy has been sent to a psychiatric hospital, and Bonnie and Rochelle lose any powers they had. They go to see Sarah and half-mockingly tell her not to be angry with them because when Sarah was made to believe that her family had died it was just an illusion. Sarah's father is packing everything in his car to move out of LA. The two girls ask Sarah if she still has any powers, and when she shows no interest in continuing a friendship with the two girls who tried to kill her, they make fun of her as they leave, saying, "She probably doesn't have powers, anyway". On hearing this Sarah makes a bolt of lightning strike causing a tree branch to fall, nearly crushing the two girls, revealing that she still has powers. As they stare back at her in shock, Sarah warns them, "Be careful. You don't wanna end up like Nancy", and smiles.
The scene cuts to a bird's eye view of Nancy's room in the psychiatric hospital. She is screaming like a maniac, telling the nurse (Esther Scott), "He gave me powers! I can fly, I'm flying, I'm flying, I'm flying!"