In a quiet Cornish village during August 1860, a mysterious plague sweeps through the community, striking residents with alarming speed. Local doctor Dr. Peter Tompson, Brook Williams, is overwhelmed and unable to halt the illness, so he sends for help from his friend and former mentor, Sir James Forbes André Morell. He arrives with his daughter Sylvia Forbes Diane Clare, a childhood friend of Peter’s wife. Together they bring a calm, careful presence to a town already frayed by fear.
As Sir James and Sylvia settle in, she steps in to avert trouble when a group of rowdy fox hunters targets a fox. Not long after, the procession for a local funeral is halted by tension and harassment from the hunters, who chafe at Sylvia’s interference. In the melee, the pallbearers topple the coffin over a bridge, revealing the corpse of John Martinus, Ben Aris a recently deceased man, a shocking omen that drifts through the village.
Seeking to understand the disease, Sir James and Peter attempt to disinter recently buried bodies and are shocked to discover some coffins are empty. Meanwhile, Sylvia is pressured and chased by the fox hunters, driving her into a manor house owned by Squire Clive Hamilton John Carson. The Squire chastises the hunters for their behavior, yet as Sylvia departs, she encounters a grey-skinned man near an abandoned tin mine headframe, who furiously hurls Alice Mary Tompson, Jacqueline Pearce the lifeless body, onto the ground. The next morning, Sylvia leads her father and Peter to the site and finds Alice’s corpse at last.
The investigation heats up as the police accuse Tom Martinus, Marcus Hammond, the brother of John Martinus, of Alice’s death; Tom protests his innocence and claims he saw his brother nearby at the mine. An autopsy reveals no signs of rigor mortis, and the blood smeared on Alice’s face turns out to be of animal origin. Sir James notices a bandaged slash wound on her wrist, a clue that deepens the mystery and points toward something more than a simple crime.
That evening, Squire Hamilton visits Sylvia and, with a calculated move, shatters a wine glass, causing Sylvia to cut her finger. Secretly, the Squire pockets a vial of Sylvia’s blood and departs, a disturbing hint of the dark ritual machinery at work. By the time of Alice’s funeral, Sylvia grows pale and faint, and Sir James and Peter uncover that Hamilton spent years in Haiti and practiced Haitian voodoo rituals and black magic, a revelation that shifts the mystery from crime to dark magic.
Hamilton now uses his voodoo knowledge to lure Sylvia into the shadowy woods, guiding her toward the abandoned tin mine with the aid of an army of walking zombies. The ceremony he plans would transform her into one of the living dead, a worker for his tin-mining scheme. It becomes clear that the squire has been turning locals into the undead to sustain the mine and profit from the ore.
Peter trails Sylvia to the perilous location while Sir James searches Hamilton’s mansion and uncovers small figures hidden in coffins—used for his voodoo rites. A fierce blaze erupts, and the room fills with smoke and fear as one servant notices the fire and alerts others. Sir James narrowly escapes and hurries to the mines to join Peter, where they confront the chaos of the ritual’s distraction.
Amid the burning, frenzied zombies, Sir James and Peter seize the moment to rescue Sylvia and flee the inferno. The scream-filled collapse of the ritual and the turmoil among the reanimated workers break Hamilton’s hold, and the town’s terror begins to lift as the plague is brought to an end. The trio’s persistence and courage leave the village with a renewed sense of safety, even as they carry the weight of the night’s unnerving revelations.
