Count Dracula arrives at the castle home of Dr. Franz Edelmann. The Count explains that he has come to Visaria to find a cure for his vampirism. Dr. Edelmann agrees to help, believing that vampirism can be cured by a series of blood transfusions. The Count agrees to this, and Edelmann uses his own blood for the transfusions. After, The Count has his coffin placed in the castle basement. That night, Lawrence Talbot arrives at the castle demanding to see Dr. Edelmann about a cure for his lycanthropy. Talbot is told to wait, but knowing the moon is rising, Talbot has himself incarcerated by the police. Inspector Holtz asks Edelmann to see Talbot, and as the full moon rises, they both witness his transformation into the Wolf Man. Edelmann and his assistant Milizia have him transferred to the castle the next morning. Edelmann tells him that he believes that Talbot's transformations are not triggered by the moonlight, but by pressure on the brain, and believes he can relieve the pressure and asks Talbot to wait while he gathers more spores from a plant he believes can cure him. Despondent by the thought of becoming the Wolf Man again, Talbot attempts suicide by jumping into the ocean, only to end up in a cave below the castle.

Edelmann searches for Talbot in the cave where they find the catatonic Frankenstein's monster, still clutching the skeleton of Dr. Niemann. Humidity in the cave is perfect for propagating the clavaria formosa, and a natural tunnel in the cave connects to a basement of the castle. Dr. Edelmann takes the monster back to his lab, but considers it too dangerous to revive him. Meanwhile, The Count tries to seduce Milizia and make her a vampire, but Milizia wards him off with a cross. Edelmann interrupts to explain that he has found strange antibodies in the Count's blood, requiring another transfusion. Edelmann's assistant Nina begins shadowing Milizia and discovers that the Count casts no reflection in a mirror. She warns Edelmann of the vampire's danger to Milizia. Edelmann prepares a transfusion that will destroy the vampire. During the procedure, The Count uses his hypnotic powers to put Edelmann and Nina to sleep and reverses the flow of the transfusion, sending his own blood into the doctor's veins. When they awake, The Count is carrying Milizia away. They revive Talbot and force the Count away with a cross who returns to his coffin as the sun is beginning to rise. Edelmann follows him and drags the open coffin into the sunlight, destroying him.

Edelmann begins to react to Dracula's blood and finds that he no longer casts a reflection in a mirror. Falling unconscious, he sees strange visions of a monstrous version of himself performing unspeakable acts. Edelmann awakens, and tries to perform the operation on Talbot. Edelmann begins leaping into a more monstrous personality, and murders his gardener. When the townspeople discover the body, they chase Edelmann, believing him to be Talbot. They follow him to the castle, where Holtz and Steinmuhl interrogate Talbot and Edelmann. Steinmuhl is convinced that Edelmann is the murderer, and assembles a mob to execute him. Talbot is cured by the operation, but Edelmann again turns into his mostrous self. Edelman revives Frankenstein's monster, with the others witnessing Edelmann's transformation, and Edelmann breaks Nina's neck and tosses 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 by accidental electrocution and Talbot shoots Edelmann dead. Talbot traps the monster under fallen shelving as a fire breaks out, and the townspeople flee the burning castle. The burning roof collapses on the monster.