Simon Merrivale owns a mask and headdress, and the legend is that whoever comes into contact with them will find their body taken over by the long-dead Shaman at full moon. Merrivale is dragged from his home, and he and the relics are taken to an isolated mansion. There, Ernest Caltro and his followers prepare to conduct a Black Mass with Merrivale as its human sacrifice. Can Simon's friends Richard Blake and Stephen Nayland mount a rescue mission?