On the night of Saturday, 23 October, in Northern California, a middle-aged man runs through an abandoned junkyard to escape two assassins. One hour later, the escapee barges into a gas station clutching a Jack-o’-lantern mask, startling the attendant, who sits watching a television advertisement for Silver Shamrock Novelties’ Halloween masks. He mutters, “They’re coming!” before collapsing. Meanwhile, Dr. Daniel Challis arrives at the house of his ex-wife, Linda Challis, after work, and presents his children with two Halloween masks. Daniel receives a telephone call instructing him to return to the hospital and attend to the collapsed man. The patient becomes erratic upon hearing the Silver Shamrock commercial jingle playing on a nearby television, claiming, “They’re going to kill us...all of us.” Once the patient is sedated, one of the assassins enters the hospital room and crushes the bones in the man’s face. Daniel chases the killer outside, but the assassin gets in his car, douses himself in gasoline, and lights himself on fire. The doctor telephones Linda and says he will be unable to pick up the children that evening. The following morning, the patient’s daughter, Ellie Grimbridge, identifies the body as that of her father, Harry Grimbridge. On Wednesday, 27 October, Daniel convinces Teddy Bryant, the hospital pathologist, to search for anything suspicious in the autopsy. On Friday, 29 October, Ellie Grimbridge runs into Daniel at a bar, and he informs her of Harry’s strange behavior upon arriving at the hospital. She brings Daniel to her father’s store and reads from his appointment book, deducing that Harry ran into trouble on his way back from retrieving an order of Halloween masks in Santa Mira, California, the location of the Silver Shamrock factory. Determined to help Ellie investigate, Daniel telephones Linda and again excuses himself from picking up his children. He and Ellie drive to Santa Mira, a secluded Irish farming community, to investigate the factory’s owner, Conal Cochran. Hoping to prevent suspicion, they rent a motel room pretending to be a married couple. While paying for the room, Daniel notices Harry Grimbridge’s name in the register. Inside their room, Daniel kisses Ellie as they argue about sleeping arrangements. At 6:00 pm that evening, speaker systems announce the beginning of city-wide curfew, and Santa Mira residents retreat to their houses. After dark, Daniel runs into a homeless man, who warns him about the town’s security cameras and declares that he plans to blow up the factory because Cochran refused to give him a job. Later, two assassins tear off the homeless man’s head. At the motel, Ellie befriends Marge Gutman, a woman who has also come to town to pick up an order from the factory. Daniel telephones Teddy, but she reports that tests have been inconclusive. When he returns to his room, he and Ellie make love. Meanwhile, Marge discovers a microchip attached to the back of one of the mask’s logo emblems. When she pokes the chip with a hairpin, a blue laser beam blasts away her mouth and nose. As she dies, a large wasp crawls from her mouth. Shortly after, a team of men in white coats remove Marge’s body from the scene, followed by Conal Cochran, who assures Daniel and Ellie that Marge will receive treatment at the factory. In the morning, Daniel telephones Teddy and asks her to find background information on Cochran. Teddy admits she is suspicious that someone has tampered with the autopsy, since she is unable to find any human remains among the ashes and automobile parts. At the factory, the workers claim they know nothing of Harry’s whereabouts after he picked up his order on 21 October. Cochran assures Daniel and Ellie that Marge is well, and invites them to accompany the family of mask salesman, Buddy Kupfer, on a guided tour of the facilities. As Daniel notices several men who resemble Harry’s killer, Ellie sees her father’s car parked in a garage, but the men stop her from approaching. That evening, Cochran’s men kidnap Ellie. Daniel attempts to make an emergency telephone call, but repeatedly receives an error message, and runs to the factory. Once inside, Daniel approaches an elderly woman kitting in a chair and demands to know Ellie’s location, but discovers that the woman is an animatronic robot. When an assassin attacks, Daniel punches him in the abdomen and pulls out a wire covered in orange slime. Just before dawn on 31 October, Cochran arrives and escorts Daniel to the “final processing” area of the factory, where he reveals a stolen piece of England’s Stonehenge monument, which he claims has powerful magical properties. He shows Daniel a video feed of the Kupfer family trapped inside a testing facility disguised as a sitting room. As the Kupfers’ son watches a flashing television image of a Jack-o’-lantern accompanied by the Silver Shamrock jingle, his latex mask melts onto his face and he collapses, as crickets and rattlesnakes emerge from his decomposing corpse. Daniel becomes lightheaded watching snakes bite and kill the boy’s parents. Meanwhile, in cities across the country, children go trick-or-treating wearing their Silver Shamrock masks. Unable to reach Daniel at the motel, Teddy telephones her colleague, and asks him to come in the following day to confirm her findings: a pile of tiny gears, washers, and springs. Just as she telephones the sheriff’s office, an assassin kills her by piercing her ear canal with an electric drill. In a darkened cell at the factory, Cochran straps Daniel to a chair and explains that the last “true” Halloween took place three thousand years earlier, when the Irish practiced sacrificial ceremonies under a particular planetary alignment. Claiming that the planets have aligned again, he hopes to use his masks and the television commercial to kill several thousand children at midnight. He places a mask on Daniel’s head and positions him in front of a television. After Cochran leaves, Daniel smashes the screen with his foot and uses the glass to cut his bindings. He escapes through a vent into the warehouse, where he uses the telephone to call Linda and warn her about their children’s masks, but she refuses to listen. He frees Ellie from her cell and sneaks into the control room, turning on the television screens and showering the workers with a box of microchips. The chips misfire, producing electric shocks that short-circuit the robotic workers and form a force field between the television sets. The Stonehenge rock emits a blue beam of light, evaporating Cochran’s body, then explodes. As Daniel and Ellie drive away, Ellie, having been transformed into a robot, attempts to break his neck, forcing him to drive off the road. Daniel knocks off her head and goes to the gas station where Harry collapsed, telephoning someone, begging to have the Silver Shamrock commercial taken off the air, and insisting that thousands of children will die. However, as Daniel desperately yells into the telephone, the clock strikes midnight and the television commercial changes to the flashing image of the Jack-o’-lantern.