Vivia, Sherry Willis-Burch, Jennifer, Joanna Johnson, and Phoebe, Elaine Wilkes, three Briggs College students, decide to pledge a sorority at Briggs College. During hell week, the sorority's housemother, Mrs. Henshaw, Pam Hyatt, urges them not to hold initiation rites at Pratt House, an abandoned fraternity nearby. She goes to Pratt House and, in an overgrown garden outside, tends Allan’s tombstone and pleads that Allan leave the students alone if they enter the house. Inside, Mrs. Henshaw is bludgeoned to death by an unseen assailant.
On initiation day, Vivia, Jennifer, and Phoebe break into a boys’ fraternity to steal clothing as part of their hazing ritual. Inside, Jennifer is confronted by Blake, Martin Hewitt, who quickly takes romantic interest in her. Back at her dorm, Jennifer is met by her shy and nerdy classmate Martin, Ralph Seymour, who returns a book she dropped outside. That night, the women arrive at Pratt House for their hazing ceremony. During the hazing, which includes the women holding raw eggs in their mouths, the lights begin to flicker and glasses start moving on their own. Vivia descends to investigate noises, and when the others follow, they find her bound to a guillotine and watching her behead. Vivia suddenly emerges from the shadows and picks up the severed head, revealing the entire sequence to be an elaborate prank using a dummy.
The following morning, Martin gives the women a ride to the sorority to learn whether they were accepted. All three are selected, though Vivia learns that her prank was the sole reason for her acceptance. Veronica, the mean sorority’s leading member, demands Vivia recreate the prank during an upcoming April Fool’s Day masquerade party at Pratt House. At a Greek-society committee meeting, Professor Zito recalls the hazing ritual 22 years ago that resulted in Allan’s death by the guillotine. That afternoon, Jennifer tells them she did research showing Allan was involved in the occult and cites news articles about people going missing in the house. Later, Professor Zito inspects the house and is electrocuted by an assailant in the basement.
At the party that night, Vivia’s prank is sprung during Veronica’s toast, and Jennifer is dragged into the basement by a hidden cord as the partygoers watch in horror. In the basement, Jennifer and Vivia laugh and quickly prepare Vivia’s guillotine stunt, when Jennifer is suddenly overcome and possessed by a spirit. She returns upstairs in silence and tells Blake she needs to leave. Vivia follows, aborting the guillotine stunt. As the party ends, Pam, a sorority sister, is killed with a trident by someone in a diving suit. The same assailant beats Veronica to death with a hammer before using the guillotine to kill Albert, Phoebe’s love interest. Blake is subsequently lured into a bathroom. Numerous partygoers are dispatched in swift succession, and Vivia discovers the dismembered body of a delivery boy named Virgil in the refrigerator.
Vivia flees upstairs to retrieve Phoebe, and the two find Blake’s drowned corpse in the bathtub, along with the bodies of other victims. They are confronted by Jennifer, who reveals herself to be possessed by Allan. Jennifer pursues them with a trident, causing the stairs to collapse as they climb. They attempt to escape through an upstairs window, but Jennifer throws Vivia to the ground, breaking her legs, before propelling after her. Phoebe climbs down the wall and begins beating Jennifer with a broken board before impaling her through the chest, killing her. Phoebe consoles Vivia, but it soon becomes clear that she has been overtaken by Allan’s spirit. Police arrive and whisk the two women away together in an ambulance, as Vivia pleads not to be left alone with the possessed Phoebe, who smiles maliciously at her. The ambulance drives away in the night, while Vivia screams.
