In the quiet town of Santa Mira, California, Dr. Daniel Challis, a recently divorced ER physician, encounters a frightened patient clutching a Halloween mask and warning of impending doom before dying mysteriously. That same night, the patient’s murderer sets himself ablaze in a shocking suicide. The victim's daughter, Ellie Grimbridge, seeks the doctor's help to understand her father's cryptic last words and they discover her father's final business trip was to the Silver Shamrock Novelties factory to pick up an order of popular Halloween masks.

Together, Challis and Grimbridge travel to Santa Mira, where they find the factory, a strange town tightly controlled by the factory’s enigmatic owner, Conal Cochran. As they investigate, they uncover disturbing occurrences linking the town and the factory to an ancient festival and a plot involving the masks. The pair soon finds themselves in a race against time, with Halloween quickly approaching, to stop Cochran’s diabolical plan. Meanwhile, the country is caught up in a Silver Shamrock commercial that is counting down to a "big giveaway" on Halloween night, featuring a jingle set to the tune of "London Bridge is Falling Down" that ominously plays on television sets nationwide.