On a quiet college campus, a young male coed is chased and killed by an unseen assailant. Meanwhile, Joanne Murray Laurie Lapinski and her boyfriend Tim Robert Fredrickson attend a campus party, discussing that their dormitory, Morgan Meadows Hall, has been condemned. Along with friends Brian David Snow, Patti Pamela Holland, Craig Stephen Sachs, and Debbie Daphne Zuniga, Joanne is staying behind during the Christmas holiday to help clear the building for its impending demolition. Tim leaves for a holiday skiing trip the next day, while Debbie reveals that she cannot stay as her parents are picking her up later in the day.
Debbie's parents arrive in the evening and wait for Debbie, who is searching for Joanne's inventory list. Her father Richard Cowgill gets impatient and leaves the car to find her, only to be bludgeoned by an unseen killer with a baseball bat spiked with nails. Her mother, Doris, Debbie's Mother Kay Beth, is then strangled in the car with a wire garrote. Debbie finds their bodies and faints in horror. The killer drags Debbie's body behind the car and crushes her head by backing over it before driving away with all three corpses in the trunk.
The next day, Patty sees a vagrant named John Hemmit Woody Roll near the campus dump site. Later that day, the caretaker, Bill Edgar Jake Jones, complains that one of his drills has been stolen, while Joanne muses that John took it. Joanne meets Bobby Lee Tremble Dennis Ely, a local salesman purchasing some of the tables from the dorm. Not long after, Bill is killed in the bathroom with the stolen drill. The following day, Craig and Brian see John walking by the dorm and try to warn him away. The group plays pool that evening but is interrupted after Patty sees John peering in at them through a window. Deciding to take matters into their own hands, the group decides to search for John around the building but is unsuccessful in finding him.
The group prepares dinner when Craig notices some of the missing food and sees John fleeing. While the group searches again for him, the killer smashes their dinner table with the spiked bat. Returning and seeing the mess, they call the police and report John. Later that night, Joanne hears footsteps on the dorm's roof and calls the others to her room when the power cuts out. On his way to the room, Brian encounters someone shining a torch in his face before he is attacked. Patty and Craig make it to Joanne's room, but Bryan does not show up, so Joanne stays behind while Craig and Patty go downstairs to reconnect the power. In the kitchen, they get separated, and Patty is grabbed from behind and knocked unconscious by the killer, who then drops her into an industrial pressure cooker and closes the lid.
Craig makes it back to Joanne's room, claiming he was knocked unconscious and that he cannot find Patty anywhere. John eventually corners Joanne while she discovers Brian's mutilated corpse in a storage room. She flees from him and makes it back to Craig. The two team up and manage to kill John. At this point, Craig reveals that he was the killer and that John knew and was trying to warn Joanne. After being chased by Craig, Joanne is shown the corpses of Patty, Debbie, and her family. Craig explains that he loves her and got rid of anyone who clung to her or ruined his chances of being with her.
Joanne attempts to flee again when Bobby appears. Craig knocks Joanne unconscious before being cornered by Bobby. The police arrive and believe Bobby is the prowler the group had reported and shoot him down when he attempts to kill Craig. The police leave to obtain reinforcements and medical help, while Craig resolves to kill Joanne. With Joanne still unconscious, Craig dumps her body into an incinerator and seemingly escapes while the police outside wonder if the smoke emanating from the incinerator exhaust vent should smell so bad.
