Sarah Bailey, a troubled beautiful teenage girl with unusual abilities, has just moved from San Francisco to Los Angeles with her father and stepmother. At her new school, she forms a friendship with a group of girls considered outcasts for one reason or another and rumored to be witches: Bonnie Harper (who bears burn scars from an auto accident), Nancy Downs (whose family lives in a trailer and whose stepfather is abusive), and Rochelle Zimmerman (who is African American and subjected to racist bullying by a group of popular white girls), who all worship a powerful deity they call "Manon". Sarah becomes attracted to the popular Chris Hooker.
When Bonnie observes Sarah levitating a pencil in class, she and the group become convinced that she is the right girl to complete their coven as "the fourth", making them all powerful. When Sarah is harassed by a vagrant with a snake (whom she had encountered before in her new house), he is immediately hit by a car and the girls believe that together they willed it to happen. It is also revealed that Sarah has attempted suicide in the past.
After a date with Chris, Sarah is upset to find that he has spread a false rumor that they had sex and she was terrible in bed. When Sarah confronts him, he treats her disrespectfully in front of his friends. In response, Sarah casts a love spell on him. In turn, Rochelle casts a revenge spell on racist bully Laura Lizzie, Bonnie casts a spell for beauty, and Nancy one for power. It very soon becomes clear that the spells have been successful: Chris becomes infatuated with Sarah, scars that Bonnie has on her back miraculously heal, and Rochelle's bully, Laura, begins to lose her hair. Nancy causes her stepfather to have a heart attack and die, enabling her and her mother to cash in on his life insurance policy and move out of the trailer park they had been living in and into a luxurious high-rise apartment.
Nancy becomes greedy for power and encourages the others to join her in a rite called "Invocation of the Spirit". On completion of the spell, she is struck by lightning. The following morning, the other girls see Nancy walking on water, with beached sharks and other dead animals littering the shore. Nancy starts acting as though she is devoid of empathy and begins taking risks with her life and those of others.
The spells that the girls have cast soon lead to negative consequences, as Bonnie becomes aggressively narcissistic, Rochelle finds Laura traumatized by her baldness and sobbing hysterically, and the obsessed Chris attempts to rape Sarah when she rejects his continual advances. In supposed retaliation, Nancy uses a glamour spell to make herself look like Sarah and attempts to fool Chris into having sex with her at a party. She is interrupted by the real Sarah, who tells Nancy to leave with her, but it becomes obvious that Nancy has unrequited feelings for Chris. Upset at being fooled, Chris says Nancy must be jealous, angering her, and she uses her power to kill Chris by throwing him out of a window.
Sarah attempts a binding spell to prevent Nancy from doing more harm, but it does not work and the coven turns on Sarah. They invade her dreams, torment her with visions of swarms of snakes, rats, and insects, and make her believe that her family has died in a plane crash. They try to induce her to commit suicide, and Nancy cuts Sarah's wrists herself. Although initially terrified, Sarah successfully "invokes the spirit" and is able to heal herself and fight back. She scares off Bonnie and Rochelle by showing them glamours in a mirror of Bonnie with her face scarred and Rochelle losing her hair like Laura, then defeats Nancy and binds her, preventing her from causing harm forever.
Bonnie and Rochelle, finding their powers gone, visit Sarah to attempt reconciliation, only to find that she wants nothing to do with them and that Manon had taken away their powers after they abused them. They scornfully mutter that Sarah must have lost her powers too; Sarah then makes a tree branch nearly fall on them. She warns them to be careful not to end up like Nancy. In the final scene, Nancy has been committed to a psychiatric hospital, delusional and stripped of her powers, and strapped to her bed as she desperately insists she can fly.
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