The story takes place at St. Mark's, a Catholic boarding school for girls. One day, a young student, Elizabeth, is studying in a classroom on the third floor, when without warning, she is attacked by unseen evil forces. One of St. Mark's priests, Father Drake, attempts to save Elizabeth from the apparently demonic aggressor, but he quickly proves to be powerless against it, and the girl vanishes without a trace.

The school is immediately shut down, and all of the students are removed from campus by worried families. Five years later, the school reopens. A harsh headmistress, Miss Pearce, rules the girls with an iron fist; Father Drake remains at the school as a teacher, but because of the disappearance of Elizabeth he has become a drunkard, found by Miss Pearce at a bar. Five troubled and unwanted girls are left by their families at the school: Alex, Mara, Cecilia — who is blind, Leah, and Connie. The girls are strictly forbidden to go to the third floor (the site of Elizabeth's disappearance). One of the girls enters the third floor, prompting Miss Pearce to get out her rod and spank the girl until she is screaming.

Alex begins to have visions of Elizabeth being possessed by a demon. Other strange things begin to happen, revealing that all five of the girls possess supernatural gifts. Eventually, the girls must use their new gifts to battle the demon Legion.

Despite a mixed reception on its release, the film has already reached a cult status and has a strong fan following.

The film has proved so popular with fans that many have started petitions for a sequel.