Hospital emergency room doctor Daniel "Dan" Challis (Tom Atkins) and Ellie Grimbridge (Stacey Nelkin), the daughter of a murder victim, uncover a terrible plot by small-town mask maker Conal Cochran (Dan O'Herlihy), who's planning a Halloween mass murder utilizing an ancient Celtic ritual. The ritual involves a boulder stolen from Stonehenge, the use of Silver Shamrock masks and a triggering device contained in a television commercial - all designed to kill millions of children.