This is a two-part episode...
On October 23, 1982, in Northern California, shop owner Harry Grimbridge, clutching a jack-o-lantern Halloween mask, is pursued by mysterious men in suits. He collapses at the shop of Walter Jones, who calls for help. Harry is taken to a hospital and placed in the care of Dr. Daniel Challis, an alcoholic doctor who has a strained relationship with his ex-wife and two children. Later that night, Harry is murdered by another suited man, who immolates himself in his car. After identifying his body, Harry's daughter Ellie meets Daniel to talk about the suspicious events surrounding Harry's death. They decide to investigate and travel to Santa Mira, California, hometown to the Silver Shamrock Novelties factory, which made the Halloween mask that Harry carried the night of his death. As they check in to a motel, Daniel learns that Harry had stayed there in the past.
Marge Guttman, another motel customer, discovers a microchip on the back of the medallion of one of the masks. The medallion emits a deadly energy beam into her mouth as she picks at it curiously with a hairpin. Her face is left mutilated, and an insect crawls out of her mouth. Shortly after, men in lab coats take Marge's body away in a Silver Shamrock van. Daniel overhears the factory technicians describing a "misfire" to factory owner Conal Cochran. While Daniel and Ellie tour the factory the following morning, Ellie finds her father's car, guarded by more men in suits who stop her from getting closer to it. They attempt to call the authorities as they flee, but Daniel cannot reach anyone outside of town by phone. Ellie is kidnapped and taken to the factory; Daniel follows and is captured by the men in suits, revealed to be androids Cochran created.
Cochran takes Daniel to the "final processing" control room and reveals his plan: the microchip on each mask contains a fragment of Stonehenge that he stole. Upon viewing the "Big Giveaway" commercial, the microchips on the masks activate, killing the wearer with brain damage and releasing a swarm of insects and snakes that kill anyone nearby.
Cochran locks a mask-clad Daniel in a room and explains his plan to resurrect ancient rituals from his native Celtic lands, sacrificing children at Samhain, the pagan celebration of the coming winter. Daniel escapes his bonds and rescues Ellie. He sneaks into the control room, activates the commercial on the screens, and pours a box of the medallions from a ceiling rafter, killing everyone. The Stonehenge remnant kills Cochran and a massive fire destroys the factory. As they flee, Daniel is attacked by Ellie, who is actually an android replacement, which he destroys after a struggle and car crash. He flees on foot to Walter's shop and frantically contacts television networks, attempting to stop the commercial broadcast. He succeeds with two networks, but seemingly fails with a third, despite screaming into the telephone, pleading with them to stop.
