Faith Ferguson is a relatively shy but intelligent teenager who is heartbroken when her mother informs her that she must spend Easter break at her Catholic girls' school instead of coming home. She's roomed with the rebellious Jamie, who initially scandalizes Faith with her wild antics. Despite her misgivings, Faith begins to form a cautious bond with Jamie and the four other teen misfits who had to remain behind — Erica, Gina, Nicole, and Kelsey, and a handsome construction worker named Daniel.
The group becomes intrigued when construction work on the school's church uncovers a Satanic temple containing the mummified remains of several schoolgirls believed to have vanished almost a hundred years ago. The teens venture into the temple one night while everyone is asleep and discover an ancient book written in Latin. Faith, fluent in Latin, translates the book and learns it describes a spell that could summon a demon from the pits of hell. Jamie and some of the others are eager to practice the spell, but Faith is reluctant due to the spell requiring a virgin sacrifice — especially after learning that the schoolgirls were murdered by a guardian devoted to keeping the spell from being cast. This tension grows as the room is found covered with graffiti warning that summoning the demon will bring death.
Jamie tries to push Faith into going along, even arranging a cruel trick by inviting Daniel to their room and making it appear as if he was trying to rape her. The girls' young schoolteacher, Sister Sherilyn, provides Faith with guidance and has her bring meals to Mother Clodah, a strange nun bearing a distinctive birthmark on her face. This ends up helping Faith avoid falling under Jamie's spell by praying with Mother Clodah, though the encounter ends with Mother Clodah's death after Jamie poisons her meal, believing Mother Clodah to be the guardian.
Jamie, who has taught herself to read Latin, presses forward with the spell and plans to use Faith as the virgin sacrifice, only to discover that Daniel is also a virgin. Horrified by the direction things are taking, Faith seeks help from Sister Sherilyn, who reveals herself to be the guardian. They manage to stop the ceremony in time to save Daniel, but at the cost of Jamie and all of the other girls involved with the spell. The outcome is grave and unequivocal: the attempted rite is halted, yet the tragedy claims the lives of the conspirators, leaving Faith to process the chilling restraint and the price of tampering with forces beyond their comprehension.