As a fierce winter blizzard blankets a secluded college town, the Kappa Sigma Kappa sorority house prepares for the upcoming Christmas break. Inside, excitement fills the air among the tight-knit
group of young women, including the sharp-witted Jess Bradford, her carefree best friend Barb Moffat, the timid and wealthy Phyl Curtis, and the house mother, the perpetually gin-soaked Mrs. Mac. However, the festive atmosphere is abruptly shattered by a series of obscene, heavy-breathing phone calls originating from inside the house itself. The caller uses a terrifying multiplicity of demented, overlapping voices, screaming obscenities and making violent threats that unnerve the residents.Unbeknownst to the sorority sisters, a deeply disturbed psychopath known only as "Billy" is lurking in their unguarded attic. Billy has brutally murdered young Clare Emerson, stuffing her lifeless body into a garment bag in the attic, marking the gruesome beginning of his holiday rampage. When Clare fails to show up for her train, her anxious boyfriend, Chris, visits the house to search for her, sparking tension. Meanwhile, local police, led by the bumbling and overwhelmed Lieutenant Fuller, dismiss the initial distress calls as a harmless prank, failing to grasp the gravity of the mounting danger.Driven by a warped psychological fixation, Billy systematically picks off the sorority sisters one by one in the shadows of the festive home. He uses a variety of improvised and brutal weapons, including a glass unicorn figurine and a plastic dry-cleaning bag, dispatching Mrs. Mac and the rebellious Barb in horrifying fashion. Despite the escalating terror, Jess remains focused on her own personal struggle: she is pregnant, and her emotionally unstable, fiercely possessive musician boyfriend, Peter, vehemently demands that she get an abortion. Peter’s erratic behavior and intense obsession make him a prime suspect in the eyes of the terrified surviving women, especially as he storms away from the house in a rage.As the body count rises and the storm rages outside, Jess finds herself trapped
alone in the dark house with the killer. In a desperate, high-stakes battle for survival, she defends herself against Billy, ultimately managing to kill Peter—whom she mistakenly believes is the intruder in the heat of self-defense. Exhausted and bleeding, Jess is heavily sedated by the well-meaning police, who leave her resting alone in her bed, blissfully unaware of the chilling truth. In the final, haunting moments, the camera pans upward to the attic above Jess’s bedroom, revealing that Billy is still alive, hiding safely in the shadows, waiting for his next strike as the phone begins to ring once more.