The isolated Louisiana village of La Foubell is struck by a mysterious, wasting plague that leaves its victims shriveled and agonized. Dr. Hugo Lamontaine, a rugged occultist and physician, concludes that the affliction is not physical, but a malignant elemental force summoned from beyond the veil to drain human vitality. Local superstition points to the swamps, where the shadowy sorcerer Aaron Kronk plots against the community.
Driven by ruthless greed, Kronk seeks to drive the villagers out so the land can be drained for a commercial enterprise. To achieve this, he uses his mediumistic powers to materialize the vaporous, life-eating horror that preys upon the weak and eventually threatens the entire populace. The terror grows stronger with each victim, stalking the darkened streets and striking fear into the simple-minded Creole inhabitants who cling desperately to their ancestral homes.
Desperate to save the woman he loves, the local schoolmaster Noel Pelletier volunteers to act as bait in Lamontaine's perilous trap. The physician prepares an elaborate defense using consecrated iron within the schoolhouse, hoping to draw out and vanquish the monstrous apparition. When Kronk attempts to intervene directly, violent clashes erupt in the gloom of the swamp's edge.
As the night unfolds, Lamontaine engages in a desperate struggle against both the intangible monster and its human master. Through sheer willpower and the protective properties of cold iron, he fights the entity inside the barricaded room while his allies lie in wait outside. The horrifying manifestation is ultimately driven back and dispersed, breaking the supernatural siege on the settlement.
With the occult threat neutralized, the conspiracy behind the terror is exposed as a land-grabbing scheme orchestrated by Kronk under an alias. The villagers are spared further devastation, and the afflicted begin their slow recovery under the doctor's care. Order returns to the swamp-side hamlet, leaving the doctor to his rum and his singular philosophy.
