 Dr. Cameron (George Zucco) has succeeded in his expierments with a serum which will turn a man into a wolf-like monster and is ready to avenge himself on the men who caused his professional failure. He uses it on his gardener Petro (Glenn Strange) and one after the other is killed by his creation. His daughter, Lenora (Anne Nagel), grows suspicious and confides with newspaper reporter Tom Gregory (Johnny Downs).