Claiborne Coe returns to the rotting, isolated village of Creston after seven years abroad, determined to marry Valyne Kirk despite a cryptic, menacing warning from his foster-father, Doctor Latham Kyle. Upon arrival, Clay senses a pervasive, stagnant evil gripping the town and discovers that Valyne now lives in Doctor Kyle's oppressive stone house. Shortly after arriving, Clay is provoked into a violent, terrifying frenzy while defending Valyne from a brutal local butcher named Jud Geer, a terrifying glimpse into a dark, uncontrollable impulse that has haunted Clay since childhood.
Invited to stay at the doctor's house, Clay encounters bizarre servants and overhears disturbing sounds emanating from a sealed cellar. The tension escalates as the household becomes convinced that a caged, inhuman monstrosity kept below is reacting to Clay's presence. Doctor Kyle eventually pulls Clay aside to reveal a horrifying family history, claiming that Clay is the twin brother of a creature born from an ancient, blasphemous ritual of demon worship conducted by their grandfather.
According to the doctor, Clay carries recessive demonic blood that threatens to turn him into a monster should he experience intense passion or marry. Consumed by horror and believing he must protect Valyne from himself, Clay contemplates suicide as the only escape from his tainted heritage. However, Valyne steadfastly refuses to abandon him, pledging her love and vowing to leave Creston with him the following morning.
The terror peaks when the monstrous entity breaks out of the cellar and abducts Valyne, carrying her up to a blood-stained stone altar on the summit of Blue Squaw Mountain. Driven by desperate urgency, Clay pursues them through the dark forest and confronts the towering, horned beast amid flickering ritual fires. In a brutal, frantic struggle, Clay fights the creature to the death, only to discover the horrifying truth when the beast is slain.
The monster is revealed to be a meticulously crafted hoax perpetrated by Doctor Kyle, who used painted wax, animal scents, and the unwitting aid of Jud Geer to orchestrate a terrifying psychological campaign. The elaborate ruse was designed to drive Clay to suicide and eliminate him so the doctor could seize Clay's overseas fortune. When Doctor Kyle attempts to murder them both to cover up the conspiracy, Clay is forced to shoot him in self-defense, finally ending the nightmare and freeing them from Creston forever.
