The plot involves a scientist who has been discredited by his peers. He attempts to kill them off after he develops a secret formula that transforms his gardener into a murderous wolf man. The story begins on a fog-bound moonlight night in a swamp; a wolf howls. The scene shifts to the nearby laboratory of Dr. Lorenzo Cameron (George Zucco), who draws blood from a caged wolf. Secured to a table is Dr. Cameron’s simpleminded but strong gardener, Petro (Glenn Strange), who is to be the doctor’s subject in an experiment. Dr. Cameron injects a serum made from a wolf’s blood into the cooperative Petro, who loses consciousness, grows fur and fangs, and awakens after he has turned into a wolf man. Dr. Cameron then turns to an empty table and visualizes his former colleagues sitting there—four professors who ridiculed his theory that transfusions of wolf blood could be used to give a human being wolf-like traits. He recalls how the scientific community, the press and the public joined in a resounding chorus of ridicule, which cost Cameron his position at the University. Addressing the spectral professors, Cameron declares, “Right now, we’re at war. At war with an enemy that produces a horde that strikes with a ferocious fanaticism”. Cameron proposes giving wolf man traits to the army to help with the war. When the professors scoff, Cameron says that his proposal matters no longer; he is now going to have his wolf man kill his former colleagues. Cameron then administers an antidote to Petro that transforms him back into a human; Petro remembers nothing. The following night, Cameron turns Petro into a wolf and sends him to the swamp. Before the night is over Petro has entered a nearby home and killed a little girl. When Cameron hears of the child’s fate, he knows his formula works. He turns to his real priority which is destroying the scientists who ruined his career. The rest of the film involves Cameron setting up elaborate scenarios in which Petro is alone with each scientist when he becomes a wolf. The more he does this the more Petro’s transformations into a wolf man become unpredictable. Cameron’s daughter Lenora (Anne Nagel) is romantically involved with Tom Gregory (Johnny Downs), a newspaper reporter who is investigating the death of the little girl. As the professors are killed off one by one, Gregory begins to suspect that Cameron is behind the slayings. The principals are in the Cameron home when a thunderstorm begins and a bolt of lightning sets Cameron’s laboratory on fire. Lenora and Tom escape from the house after encountering Petro in wolf form. Petro turns on Cameron and kills him, just before the fire brings the house down on both of them.
