The narrative follows a young hospital interne, Dr. Haverill, who accepts a three-month substitution for a country doctor in a secluded backwater district of Pennsylvania. Upon his arrival, he finds his predecessor, Lennox, prematurely aged, deeply troubled, and anxious to flee. Haverill settles into the quiet routine of the village, but soon becomes entangled in local mysteries, notably the strange nocturnal attacks suffered by an eccentric researcher named Lessing, who lives in an isolated bungalow near a ruined saw-mill. Lessing bears deep, claw-like lacerations on his body and insists he is being hunted by an unknown entity, which he connects to the legendary 'Jersey Devil' and a series of unexplained cyclist fatalities on nearby lonely roads.
As Haverill delves deeper into the local gossip and encounters the sullen, hostile Pennsylvania Dutch inhabitants—specifically the chicken huckster Aaron Menning and his mute, uncanny mother—he realizes that human malice is intricately woven into the eerie local legends. The atmosphere thickens with dread, haunted by tales of 'Crazy Jake,' Aaron's supposedly deceased half-brother, whose ghost is said to roam the foggy precincts of Dutchman's Hill. When Haverill's close friend Dr. Herrick arrives for a holiday, the two physicians begin a systematic investigation that cuts through the local superstitions and points toward a horrific reality.
The mystery deepens with the return of Lennox under an assumed identity, acting as a secret investigator, and culminates in a terrifying night chase through the dark woods near the saw-mill. The dark secret of the Menning household—rooted in a monstrous circus legacy and profound congenital degeneration—is finally dragged into the light. In a shattering climax at dawn, the true nature of the predator that has terrorized the countryside is revealed, bringing a violent end to the dark shadow that hung over the remote valley.
