NOTE: Sequel to "House of Frankenstein" (1944)
Dr Franz Edelman [Onslow Stevens] of Visaria has a reputation for treating patients with strange diseases. Consequently, Count Dracula [John Carradine] (known to the outside world as Baron Latoes) and Larry Talbot [Lon Chaney, Jr] (aka The Wolfman) come to Edelman in search of cures for their problems. After securing a blood sample from Dracula, Edelman finds that his blood contains an unknown parasite. Edelman thinks he can create an antitoxin and administer it through a series of blood transfusions. In the meantime, Dracula can sleep in his coffin in the basement.
Talbot doesn't fare as well. When he calls, Edelman is unable to see him, so Talbot races to the village jail and asks to be confined in a cell. When Edelman arrives, he is able to personally witness the transformation of Talbot into the Wolfman and agrees to attempt a cure. After examining Talbot, Edelman announces that pressure on his brain is causing his glands to secrete an abnormal supply of the hormone that causes him to transform. Fortunately, in an attempt to help his nurse Nina [Jane Adams] become cured of her hunchback, Edelman has been working on a mold which softens calcium salts (i.e., bone). This mold might also be used to reshape Talbot's skull and take some of the pressure off his brain.
Unfortunately, it will take at least a month to produce enough of the mold for the operations. Distraught at the thought of becoming the Wolfman one more time, Talbot attempts suicide by jumping from a cliff. Thinking that the rushing river below might have washed Talbot into some nearby caves, Edelman is lowered into the caves where he discovers perfect conditions for growing the molds and convinces Talbot to return for treatment. He also discovers the body of the Frankenstein monster [Glenn Strange] as well as a secret passageway between the cave and his laboratory.
In the meantime, Dracula has become smitten with Edelman's other nurse, the luscious Melissa Morelle [Martha O'Driscoll], and tries to entice her into joining him. But Nina oversees the Baron casting no reflection in a mirror and reports this observation to Edelman who rushes to the garden in time to save Melissa. Edelman tells Dracula that another transfusion must be done immediately. This time, however, Dracula reverses the blood flow so that some of his blood contaminates Edelman. When Edelman realizes what Dracula has done, he moves Dracula's coffin into the sunlight and destroys him, releasing the hold on Melissa.
But now Edelman starts transforming. During the day he is himself, but at night he surges with the urge to kill. Knowing that his time is limited, Edelman operates on Talbot. That night, from his wheelchair by the window, Talbot sees Edelman hop a cart. Edelman subsequently kills the cartdriver, and the villagers follow him home thinking that he is Talbot. The police interview both Edelman and Talbot but agree that Talbot is physically incapable of such activity. Talbot has put two and two together and promises Edelman that he will say nothing until Edelman is able to operate on Nina.
That night, however, Edelman reawakens the Frankenstein monster. Nina tries to stop him, but Edelman strangles her. Talbot shoots Edelman, and the Frankenstein monster turns on Talbot and begins destroying the laboratory. The monster is killed in a fire, and Talbot walks off into the moonlight with Melissa. [Original synopsis by bj_kuehl.]