A week before Halloween, an older man named Harry Grimbridge (Al Berry) is wounded by a mysteriously dispassionate group of assailants in an industrial parking lot. After receiving treatment at a local hospital from Dr. Dan Challis (Tom Atkins) -- a hard drinking, divorced father of two -- Grimbridge is killed by an assassin who later sets himself on fire. Blowing off his own kids, Challis teams up with Grimbridge's daughter, Ellie (Stacey Nelkin), to find out why the middle-aged toy salesman was murdered.

The duo's search soon leads them to a Halloween-mask factory run by inventor and practical joker Conal Cochran (Dan O'Herlihy). In between bouts of passionate lovemaking, Ellie and Challis begin to realize that the sinister old businessman has something other than treats in mind for America's kids -- something to do with the Silver Shamrock masks that Challis' children and thousands of other youngsters have bought for the holiday.

Original screenwriter Nigel Kneale, whose scripts for Britain's Quartermass TV series made him a beloved science fiction fixture, sued the producers of Halloween 3 to have his name removed from the credits after seeing the gory finished product; director Tommy Lee Wallace ultimately received screenplay credit. John Carpenter, director of the first two Halloween films, co-produced the third installment with Debra Hill, who would later soldier on without Carpenter for additional, belated sequels.