An apparent murder-suicide in a hospital emergency room leads to an investigation by the on-call doctor, which reveals a plot by an insane toymaker to kill as many people as possible during Halloween through an ancient Celtic ritual involving a stolen boulder from Stonehenge and Halloween masks.

The original writer of the story was Nigel Kneale but he sued the producers to take his name off the movie after seeing how violent it was.

Conal Cochran:
You don't really know much about Halloween. You thought no further than the strange custom of having your children wear masks and go out begging for candy.

Continuity: Towards the end of the movie, Chaliss's shirt goes from untucked to tucked several times between shots.