A young couple, Harry and Valerie Spalding, relocate to the remote village of Clagmoor Heath after inheriting a cottage from Harry's deceased brother. Upon arrival, they notice the villagers' fearful and superstitious behavior, coupled with abnormally high incidences of unusual deaths. These deaths are characterized by grotesque physical symptoms, particularly a blackened mouth with foamy discharge. Curious and disturbed, Harry and Valerie begin to investigate the mysterious circumstances surrounding these fatalities. They befriend Mad Peter, an eccentric local who shares cryptic warnings about the dangers lurking in the village.

As they delve deeper into the mystery, Harry encounters Dr. Franklyn, a reclusive and enigmatic scholar living in a nearby manor with his daughter Anna. Dr. Franklyn's behavior is notably aloof and secretive, raising Harry's suspicions. Meanwhile, Valerie forms a tentative friendship with the seemingly fragile Anna, who is kept under tight constraints by her father. Strange occurrences and ominous signs escalate, leading Harry to unearth a horrifying secret tied to Dr. Franklyn's past expeditions. The sinister truth threatens both his and Valerie's lives as they find themselves entangled in a deadly curse that plagues the village.