Dr. Cameron injects a serum consisting of wolf's blood in his simpleminded gardener Petro. Petro begins to transform into a wolf-man. Seeing his experiment succeed, Dr. Cameron reflects on his colleagues who dismissed his theory and how the scientific community, the press, and the public joined in a resounding chorus of ridicule that finally cost him his position at the university.
Dr. Cameron uses Petro to kill. After he kills a young girl, he knows he will follow through with killing his enemies. However, each time he causes Petro to transform, he becomes less predictable. On the last time he is transformed, Petro turns on Dr. Cameron. Lightning strikes his home, setting it ablaze. Petro attacks Dr. Cameron but both perish in the fire.