 In his nearby laboratory, Dr. Lorenzo Cameron (George Zucco) 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 subject of the doctor's experiment.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 transformed into a wolfman.