HALLOWEEN III Season of the Witch takes a break from the Michael Myers storyline and concentrates on a small California town called Santa Mira. There, a large mask-making company called Silver Shamrock has opened up, selling popular Halloween masks to the entire country's children. Their advertising covers the media with rhymes, set to the tune of "London Bridge is Falling Down," which chant "Two more days 'til Halloween, Halloween, Halloween..."

Dr. Dan Challis, a local doctor, stumbles on a plot to bring about the end of human kind. He, along with Ellie, a young woman who's father was recently killed, travel inside the factory, where they meet Cochran, an old irish goon who has stolen a rock from Stonehenge, which causes mutations in the person who wears a Silver Shamrock mask! So tonight, on the most deadly Halloween ever, children across the nation will be watching Silver Shamrock commercials, which cause the mutations - their only hope is Dr. Challis and Ellie!

However, Cochran captures both Ellie and Challis. The doctor is well aware of the devestation of Cochran's plot...so he escapes and with the help of Ellie, he leaves the control center exploding in bursts of flames. He races to get help and to stop the commercials from airing. But, Cochran will have the final Halloween trick of the night.