-On Saturday October 23, shop owner Harry Grimbridge(Al Berry) is being chased by mysterious men in gray suits. He collapses at a service station holding a Silver Shamrock jack o lantern mask and is driven to the hospital by the station attendant(Essex Smith), while saying "They're going to kill us all" He is placed in the care of Dr. Daniel Challis(Tom Atkins) Another man(Dick Carelock) in a gray suit enters his room and kills him by gouging out his eyes. And when he leaves, a nurse discovers Grimbridge's dead body and screams, alerting Dr. Challis. He runs outside to find the man in his car, who kills himself by pouring gasoline on himself and lighting up, blowing himself up.
-Dr. Challis then meets with Grimbridge's daughter Ellie(Stacey Nelkin), which leads to them investigating it. It leads them to a small town in California called Santa Mira, which has mostly Irish immigrants, and the Silver Shamrock Novelties factory. They check into a hotel run by a man named Rafferty(Michael Currie) who reveals that the source of the town's prosperity is Irishman Conal Cochran(Dan O'Herlihy) and that the majority of the town are descendants of Irish immigrants. Challis learns that Ellie's father stayed at the same hotel while picking up more Halloween masks. Soon, other guests arrive, also business owners, which include Marge Guttman(Garn Stephens), Buddy Kupfer(Ralph Strait), his wife Betty(Jadeen Barbor) and their son Little Buddy(Bradley Schacter). All of them have business at the factory and eventually meet bad ends through the Silver Shamrock masks.
-Challis and Ellie tour the factory posing as tourists with the Kupfers and are alarmed to find Grimbridge's car there and more suspicious looking men in suits. They return to the hotel but can't contact anyone outside the town. Ellie is kidnapped by the men in suits. He breaks into the factory to find her and discovers the men in suits are actually androids created by Cochran. Challis is captured by the androids and Cochran reveals his plan to kill children on Halloween night. The Silver Shamrock trademark on the masks contain a fragment of Stonehenge. When the Silver Shamrock TV commercial airs on Halloween night, the chip will activate, killing the wearer and unleashing a lethal swarm of insects and snakes, killing those around the wearer. Cochran explains that his plan is to resurrect macabre aspects of Gaelic festival Samhain, which is connected to witchcraft