On a cold winter night in the rural town of Mountaincrest, a disheveled man in a suit wanders down the main street and slips into a bar, insisting he has no idea who he is or where he came from. In the tense hush that follows, the local troublemaker, Joe Mitchell, demands money as the man exits. A struggle spills into an alley, and Ma Everett [Jean Harvey] glimpses four legs on the sidewalk and hears a savage snarl in the darkness; a figure surges from the shadows and disappears into the night.
Sheriff Jack Haines [Don Megowan] soon has Joe’s body brought to Dr. Jonas Gilcrist [Ken Christy] and nurse Amy Standish [Joyce Holden]. Gilcrist notes the wounds look animal in origin, yet Ma Everett’s horror story of “a thing” rather than a beast lingers in his mind. A posse is formed in response, determined to find the creature prowling Mountaincrest.
Deputy Ben Clovey [Harry Lauter] is attacked by the thing, and he describes it in halting, breathy detail: perhaps hands covered with hair, or paws like a wolf, but not entirely wolf. Jack declares a werewolf has claimed another victim, and Gilcrist and Amy walk the line of belief with him.
The disheveled man returns to Gilcrist’s office, unable to recall anything after a car accident. He confesses that he killed Joe, then flees when Amy tries to administer a sedative, crying out, “Those other doctors did something to me!” > Those other doctors did something to me!
Across town, Dr. Morgan Chambers [George Lynn] and Dr. Emery Forrest [S. John Launer] reveal their startling experiment: they injected the man with an experimental “irradiated wolf serum,” a prototype they hope will one day let a small, preselected group survive a looming nuclear holocaust, with lycanthropy as an unfortunate side effect. The man’s wife, Helen Marsh, and their young son Chris Marsh arrive and identify him as Duncan Marsh. The doctors, fearing blame, press toward Mountaincrest with a chilling plan—to kill Duncan before he can transform again.
In a mineshaft, Forrest corner’s Duncan, who pleads for mercy before he inevitably changes. The monstrous shift comes quickly: he becomes a werewolf and attacks Forrest, only to be driven back by shots from Chambers.
Amy urges Jack to bring Duncan in alive, and the group—Helen and Chris, along with Ben—set out to help. Helen calls Duncan through a megaphone, and in his human form he approaches, tearfully embracing his wife and son. Yet he begs Amy to take them away, fearing another turn could cost them everything they hold dear.
Duncan is placed in a jail cell, while Chambers and Forrest push their dangerous plan forward, attempting another injection. The werewolf materializes again, killing the two doctors and vanishing into the forest night.
The chase culminates on a bridge, where the werewolf is shot dead by the waiting posse. As the life drains from the creature, the beast reverts to Duncan’s human form, leaving the town to reckon with the hidden costs of science, fear, and a night that revealed a brutal truth about what humanity might unleash.
