In a grim, holiday-tinged opening, a man in a Santa suit and a woman meet in an alleyway to share a private moment in a car, only to be stabbed to death by a killer wearing a grinning translucent mask. The shocking violence continues at a festive party, where another man dressed as Santa is killed when a spear is driven through his head, witnessed by his daughter Kate Brioski. The scene shifts to Scotland Yard, where Chief Inspector Ian Harris, [Edmund Purdom], and Detective Sergeant Powell [Mark Jones] discuss the string of brutal murders and begin interviewing Kate and her boyfriend Cliff.
The next day, a present arrives for Harris with the ominous message, Don’t Open Till Christmas, while Powell receives a cryptic phone call from a man claiming to be a reporter named Giles. A Santa is shot in the mouth, adding to the growing sense that the killer is targeting people in the Christmas tableau. Cliff manipulates Kate into visiting a porn studio owned by an old friend; when Kate storms off, Cliff stays behind as the model in a Santa cloak prepares for outdoor photographs, but Cliff bolts when police officers near by, and the model narrowly escapes the killer.
At a peep show, a Santa is stabbed, an event witnessed by the stripper Sherry Graham. Harris then visits Kate and Cliff and makes it plain that Cliff is a suspect due to his presence at multiple attacks. Powell notices Giles creeping through his own office, and notes that the newspaper Giles claimed to work for denies any recollection of him. Giles retorts by insinuating that Harris hides something, urging Powell to keep a vigilant eye on him. A Santa is assaulted by a group of teenagers and flees into the London Dungeon, where he and an employee are killed.
In a bid to trap the killer, undercover officers don Santa disguises, but two of the undercover agents are butchered at a carnival. The killer abducts Sherry, intending her as “the supreme sacrifice to all the evil that Christmas is.” Meanwhile, Harris is removed from the case, and when Kate calls him, she is told by his housekeeper that he is visiting Parklands, a mental institution. A Santa chases into a theatre where Caroline Munro, [Caroline Munro], is performing, and his body is wheeled onto the stage via a trapdoor after a machete blade ends his life.
Kate shares her suspicions about Harris with Powell, but he dismisses the idea since Harris has no birth certificate. Undeterred, she visits Parklands alone. A separate killer castrates a Santa in a department-store restroom, heightening the sense of menace. Kate is confronted at home by Giles, whom she has learned was just released from Parklands and who is revealed to be the younger brother of Harris (who had changed his surname from Harrison after Giles’s commitment). Powell phones Kate, who tries to answer, but Giles strangles and stabs her. Powell rushes to Kate’s apartment only to be pursued by Giles into a junkyard, where Giles electrocutes him.
Giles returns to his hideout and chases Sherry through the space when she escapes her chains. Sherry knocks Giles over a railing, and when she investigates the body, Giles suddenly revives and begins throttling her. A flashback reveals decades earlier Giles walked in on his father, who was dressed as Santa for a Christmas party, cheating on his wife with another woman. When Giles’s mother discovered the affair, she and her husband argued, ending with Mrs. Harrison being knocked down the stairs.
Harris awakens from a nightmare, goes into his living room, and unwraps a present that carries a previously unseen card reading “Christmas present from your loving Brother.” The gift houses a music box, which detonates with a startling explosion as it begins to play.
