Four years after the events of The Wolf Man and The Ghost of Frankenstein, two graverobbers break into the Talbot family crypt under a full moon and steal the wolfsbane buried with Larry Talbot. As the moon rises, his body is revived and transformed into the Wolf Man, a terrifying return that sends shockwaves through Cardiff. He is found that same night by the police, still bearing a fatal head wound, and brought to Cardiff Hospital where Patric Knowles plays Dr. Frank Mannering, who takes charge of his care.
During the first full moon since his inexplicable revival, Larry slays a police constable, forcing the doctor to confront a case that seems impossible to explain. The next morning, Larry remembers everything and pleads with Mannering to summon the police before he can strike again. Inspector Owen, suspicious and having been told of a supposed death, demands to know Larry’s true identity, while Larry’s temper flares into violent irateness. The helpless man is restrained by orderlies and strapped to a bed, leaving him trapped in a hospital that can’t easily contain a creature of moonlit transformation.
Not convinced by the supernatural explanation, Mannering and Owen decide to investigate the Talbot tomb in Llanwelly, only to discover that Larry’s body is gone and that one of the grave robbers has been slain in the same manner as four years earlier. In a desperate bid to escape, Larry bites through his leather bindings and breaks free, vanishing into the night. He travels far from Wales in search of a cure for his seemingly undying condition and seeks the wisdom of the gypsy Maleva, Maria Ouspenskaya, who believes the key to his fate might lie beyond ordinary medicine and hints that the path could involve ancient science.
Maleva guides him toward Europe, toward the village of Vasaria, where Larry hopes to find Ludwig Frankenstein’s notes in the remnants of the estate. To his surprise, he encounters Frankenstein’s Monster, Bela Lugosi, trapped in ice and frozen in time. Larry frees the creature, hoping the notes might help illuminate a way to cure him, but the search leads him into a delicate web of deception and desire. To gain access to the secrets, Larry poses as a prospective buyer of the Frankenstein estate and draws the attention of Baroness Elsa Frankenstein, Ilona Massey the daughter of Ludwig, who is wary of him but intrigued by the possibilities the notes may hold.
Elsa declines to aid him at first, but she and Mannering are soon drawn into a larger plan after an invitation to the Festival of the New Wine from the Burgomeister. When Mannering finally catches up to Larry, he urges him to seek institutionalization before another surge of danger erupts. The festival becomes a battleground when the Monster crashes the celebration and is attacked by angry villagers. Larry, in a rare moment of mercy, drives the Monster to safety on a waiting cart.
The next morning, Elsa, Mannering, and Maleva reunite with Larry and the Monster at the ruins. Elsa receives the crucial notes from the Baroness, and Larry asks that his life energy be transferred to the Monster in the hope that the laboratory can be repaired. Mannering agrees, but the plan’s details are compromised when he and Elsa secretly intend to drain both Larry and the Monster of life.
The procedure begins, but Mannering’s curiosity compels him to push the experiment to its limit, reviving the Monster fully. The coincidence of the full moon heightens the tension as both the Wolf Man and the Monster regain strength. The Monster tries to seize Elsa, prompting a fierce confrontation with the Wolf Man. Vazec the Innkeeper, Rex Evans, grows suspicious of the conspiracy surrounding Larry and the Monster and detonates dynamite to destroy the dam above the Frankenstein estate, hoping to drown everyone involved. Elsa escapes with Mannering, but the epic clash between the two creatures culminates in a flood that sweeps them all away, leaving the fate of the Wolf Man and the Monster in the hands of the raging waters and the ruined estate.
