TO ALL A GOODNIGHT

During Christmas vacation at the rural Calvin Finishing School For Girls, a student is killed when she is accidentally pushed over a balcony during a prank. Two years later, on the Friday before Christmas, the school is emptying out for the holiday; however, five students — Nancy, Melody, Leia, Trisha, and Sam — decide for various reasons to remain at the school, planning to have a weekend get-together with their respective boyfriends. That night, while the remaining girls have dinner, their classmate Cynthia and her boyfriend are murdered outside the school by an assailant with a hunting knife.

The others meanwhile coerce the innocent Nancy into giving their housemother, Mrs. Jensen, milk laced with sedatives to make her fall asleep. After she falls asleep, the girls go to a nearby airstrip to meet their boyfriends, T. J., Alex, Tom, and Blake, who have flown in on a private plane. At the house, the group sit in the living room and talk. Trisha goes to the kitchen to retrieve beer, but is confronted by someone in a Santa Claus suit and mask, who slits her throat. When Trisha does not return, Tom goes to find her. He is confronted by the killer, who chases him outside and smashes his head with a rock. The killer buries each body in the school's garden.

Later, Sam and Blake have sex in the parlor. They are interrupted by the killer, disguised in a decorative suit of armor, who shoots Blake with a crossbow and decapitates Sam with an axe. Meanwhile, Nancy runs into the school's groundskeeper, Ralph, who ominously tells her he believes something evil is about to happen. Melody, meanwhile, seduces Alex in her bedroom and gives him a handjob.

The next morning, Nancy finds Ralph's corpse in the woods outside. A detective, Polansky, investigates the murder, and suggests the students remain sequestered inside for the remainder of the weekend. He also surmises that those missing may be victims of the killer, or possibly perpetrators in Ralph's death. That night, Jim, a police officer stationed outside the school, is murdered by the killer with an axe. Inside, Leia seduces another officer named Dan before going to take a shower. In the bathroom, she finds Sam's severed head hanging from the shower head. Dan rushes into the bathroom, but is stabbed to death from behind as he enters the doorway. Nancy and Alex subsequently come upon the scene, and find Leia suffering a psychotic break, traumatized by what she has witnessed; unable to speak, she begins dancing and humming to herself.

Melody and T. J. meanwhile are outside, unaware of what has happened. They kiss under a tree, but the killer, hiding above, strangles T. J. to death with a garrote made of wire. Melody flees inside to find Nancy, Alex, and Leia; the four are confronted by the killer in the Santa outfit, who reveals themselves to be Mrs. Jensen, avenging the death of her daughter who was killed in the prank two years prior. While Mrs. Jensen stalks Nancy through the house, Melody flees back to the airstrip and begs the pilot, sleeping beneath the plane, to take her to safety; the two are killed, however, when an unknown person in the Santa costume starts the engine, slicing them to pieces with the propeller.

Back at the school, Nancy is chased by Mrs. Jensen to the balcony. In a struggle, Mrs. Jensen is thrown over to her death. A horrified Nancy goes downstairs, where she is confronted by a second assailant in a Santa costume; it is revealed as Polansky, who explains that he is Mrs. Jensen's husband. Polansky attempts to strangle Nancy, but is killed with a crossbow by Alex. Together, Alex and Nancy flee the school, leaving Leia behind, dancing maniacally on the balcony, singing to herself.

The killer, revealed to be Mrs. Jensen avenging her daughter's death, stalks and confronts the surviving students in the school.

The film resolves its central mystery by connecting the present-day killings to the fatal prank from two years prior, with Mrs. Jensen's vengeful rampage serving as retribution. This revelation ties the holiday horror back to its moral roots, delivering a cautionary tale about the consequences of youthful recklessness and the dangers lurking beneath supposedly safe traditions.