A mad scientist's experiments turn an innocent man into a nocturnal killer.

Scientist Lorenzo Cameron perfects a transfusion serum using the blood of a wolf and a human. When he injects his simpleton hired hand Petro with the serum, the man transforms into a wolfman, but has no recollection of his actions after Cameron administers an antidote. One night Cameron releases Petro into the nearby swamp after his transformation, and the wolfman murders an innocent child. Cameron plans to use Petro to seek revenge against his former university colleagues, who sought his dismissal when he insisted on pursuing his heretical theories. Cameron retreated to the swamp area with his daughter Lenora, who has no knowledge of the nature of her father's experiments and remains reluctantly because he insisted she leave the city without a word to her boyfriend, reporter Tom Gregory. Cameron's first intended victim is Professor Blaine, a friend of Tom. Tom is stunned by Blaine's death, which appears to have been caused by an animal, and, remembering the news story about the death of the child by a strange animal, Blaine goes to the swamp area and discovers Cameron's residence. Lenora is thrilled to see Tom, but her father resents the intrusion and insists that he leave. Tom becomes lost in the swamp and finds refuge at the murdered child's home, where he learns from the locals about the murderous animal that stands on two legs. The next night Cameron's former colleague, Fitzgerald, visits him, and Cameron proposes to show him his experiment. Fitzgerald is horrified that Cameron has followed through with his insane theories and leaves angrily, but agrees to give Petro a ride into town. Petro transforms during the drive and attacks Fitzgerald. The car crashes and Petro carries Fitzgerald into the swamp, but drops him and flees when Tom, with a group of locals, spots him. They bring Fitzgerald to the Cameron home, hoping that he will revive long enough to name his assailant. However, Cameron murders Fitzgerald before he can speak. Lenora wanders into the secret chamber that hides her father's laboratory and unwittingly releases Petro, who has wandered in from the swamps. Her screams bring Tom and Cameron to her aid and they escape harm. However, when a lightning strike starts a fire in the house, Petro goes on a rampage and murders Cameron. Tom and Lenora escape outside, and her father and Petro burn to death with the house.