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 and stepmother. She enrolls in a local Catholic high school, but has trouble fitting in. During a class, her classmate Bonnie (Neve Campbell) witnesses Sarah, with her mind, 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 and abusive stepfather, Bonnie has massive scars all over her back, and Rochelle is subjected to racist taunts by popular girl 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 bum, and all four girls simultaneously will for something to happen; the bum is 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. Nancy, Bonnie and Rochelle comfort Sarah, and invite her on a field trip. They take the bus out to the country where 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, Bonnie's scars miraculously heal, and the next time Laura bullies Rochelle, Laura's hair begins falling out. At home, Nancy's abusive stepfather has a heart attack and dies, then Nancy and her mother find out that they have inherited $175,000 from an insurance policy.
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 girls witness Nancy walking on the water, and from this point on, Nancy's powers have increased.
Sarah's love spell goes too far when Chris attempts to rape her. Nancy tries to seduce Chris by disguising herself as Sarah, but when the real Sarah arrives, Nancy causes Chris to fall out a second-story window, killing him. Sarah tries to stop Nancy by binding her powers, but this does not work and the three girls abandon Sarah. 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 talent has passed on to Sarah.
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. Nancy tells Sarah that in the old days, if a witch betrayed her coven they would kill her. Nancy later slashes Sarah's wrists. Sarah runs to her room, while Nancy, Bonnie and Rochelle wait for her to die. Sarah is almost too weak to invoke Manon, but then she hears her mother's voice whispering, "Don't be afraid. Reach inside yourself." Sarah successfully invokes Manon and is able to cast spells to fight back. Forced by a counterspell of Sarah's to see themselves and what they've become by catching their reflections in a mirror, Bonnie and Rochelle flee, leaving only Nancy and Sarah. Nancy and Sarah have a showdown, in which Nancy is ultimately defeated by Sarah, who 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 asking if she still has any powers, and when she shows no interest in continuing a friendship with two girls who tried to kill her, they make fun of her as they leave. On hearing this Sarah makes a bolt of lightning strike causing a tree to fall, nearly crushing the two girls, revealing that she still has powers. She warns them to "be careful, you don't wanna end up like Nancy.". The scene cuts to a birds eye view of Nancy's room in the psychiatric hospital. She is screaming like a maniac, telling the nurse "He chose me, I can fly,". The credits roll.
Robin Tunney as Sarah Bailey