Count Dracula, John Carradine arrives at the castle home of Dr. Franz Edelmann, seeking a cure for his vampirism. Dr. Edelmann, convinced that a series of blood transfusions using his own blood might heal him, agrees to help. After the procedure, the count’s coffin is placed in the castle basement, setting the stage for a tense alliance between science and immortality.
That night, Lawrence Stewart Talbot, Lon Chaney Jr. arrives at the castle to seek a cure for his lycanthropy. He is told to wait, but with the full moon rising, he takes matters into his own hands and has himself incarcerated by the police. Inspector Holtz, Lionel Atwill asks Dr. Edelmann to see Talbot, and as the moon climbs, they witness Talbot’s dramatic transformation into the Wolf Man. Edelmann and his assistant Miliza Morelle, Martha O'Driscoll, arrange to have him transferred to the castle the next morning. Edelmann insists that the moonlight may not trigger the change after all, arguing that the real trigger is pressure on the brain, and he vows to keep Talbot waiting while he searches for a cure using plant spores. Despair over the prospect of becoming the Wolf Man again drives Talbot to a desperate act: he attempts suicide by leaping into the ocean, only to find himself trapped in a cave beneath the castle.
In the cave, Edelmann finds Talbot and they stumble upon the catatonic Frankenstein’s Monster, still clutching the skeleton of Dr. Niemann. The cave’s humidity proves ideal for Clavaria formosa, and a natural tunnel connects to a basement in the castle. Edelmann considers reviving the Monster but judges the plan too dangerous to pursue, even as the discovery broadens the scope of the doctor’s experiments.
Meanwhile Dracula tries to seduce Miliza Morelle, Martha O'Driscoll; she wards him off with a cross. The doctor interrupts to explain he has found strange antibodies in the Count’s blood, which will require another transfusion. Nina, Jane Adams, shadows Miliza and discovers that Dracula casts no reflection in a mirror. She warns Edelmann of the vampire’s danger to Miliza, and the doctor prepares a transfusion that he believes could destroy the vampire. During the procedure, Dracula uses his hypnotic powers to put Edelmann and Nina to sleep, then reverses the flow of the transfusion, sending his own blood into the doctor’s veins. When they awaken, the count is already carrying Miliza away.
Edelmann and Nina regain their footing and force Dracula back with a cross; the vampire crawls into his coffin as the sun rises. Edelmann follows and drags the open coffin into the sunlight, destroying him. But the trouble does not end there, for the doctor’s own blood begins to alter him in unseen ways, and he discovers Dracula no longer casts a reflection in a mirror. A cascade of visions—of a monstrous version of himself performing unspeakable acts—haunts Edelmann as he tries to proceed with Talbot’s treatment.
Back in the castle laboratory, Edelmann attempts to perform the operation on Talbot, but the strain of the transformation re-emerges in him. He becomes a more sinister, monstrous personality and murders his gardener. When the townspeople discover the body, they chase Edelmann, mistaking him for Talbot. The mob follows him to the castle, where Holtz and Steinmuhl confront Talbot and Edelmann. Steinmuhl closes in on the doctor, convinced that the physician is the killer and organizing a mob to execute him. Meanwhile, the operation finally cures Talbot, but Edelmann’s monstrous self resurfaces with even greater ferocity.
In a frenzy, Edelmann revives Frankenstein’s Monster, and the others witness his terrifying transformation. He wreaks further harm, breaking Nina’s neck and throwing her body into the cave. Holtz and Steinmuhl lead the townspeople to the castle, where the police attack the monster but are subdued by the creature. Edelmann kills Holtz in an accident—electrocution from a loose wire—before Talbot shoots Edelmann dead. Talbot then initiates a fire to trap the monster, and as the castle burns, the townspeople flee. The collapsing roof seals the Monster’s fate, ending a night of science, superstition, and escalating horror.
