Dr. Daniel Challis and Ellie Grimbridge stumble onto a gruesome murder scheme when Ellie’s novelty-salesman father, Harry, is killed while in possession of a strange mask made by the Silver Shamrock mask company. The company’s owner, Conal Cochran, wants to return Halloween to its darker roots using his masks — and his unspeakable scheme would unleash death and destruction across the country.

Kate’s husband Richard hasn’t seen his cousin Harry for 20 years. So when he comes to stay with his fiancee Connie they are welcomed. But Kate senses her family is at risk when they start behaving suspiciously.