The plot involves a scientist who has been discredited by his peers. He plans to kill them off by using a secret formula that has actually transformed his gardener into a wolf man.

The story begins on a fog-bound moonlight night in a swamp; a wolf howls. The scene shifts to a room where another wolf is being kept in a cage by Dr. Lorenzo Cameron (George Zucco), who expresses his understanding that the wolf would rather be out howling at the moon like the other wolf is but he adds that this caged wolf is serving the cause of science. Dr. Cameron walks over to the other side of the laboratory where his uneducated but strong gardener, Pietro (Glenn Strange) is strapped to a couch. After talking sweetly with the gardener, Dr. Cameron gives him an injection. Pietro succumbs to slumber and a transformation occurs: he grows fur and fangs and develops a vicious, snarling personality. Cameron row regards him as a Wolfman and no longer a human being.

Dr. Cameron then turns to an empty table and visualizes his four former colleagues sitting there—the Professors Blaine, Fitzgerald, Warwick and Hatfield. Dr. Cameron recalls how they ridiculed his theories of using blood transfusion to give a human being wolf-like traits, and how the scientific community, the press and the public joined in a resounding chorus of ridicule, which cost Cameron almost everything that mattered to him. Most notably, he lost the esteem of the scientific community and 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 to giving these wolfman traits to the entire army. He feels that an army of such creatures would be invincible. This suggests the story is taking place during World War II. The images continue to argue and Cameron points out what Pietro has become is real. He then says that his original goals matter no longer; he is now going to having his wolfman killed by each one of his former colleagues.

The scene ends when Cameron gives Pietro the antidote, transforming him back into a human. Pietro wakes up not remembering anything and Cameron sends him on his way. Cameron leaves the laboratory and visits his daughter Lenora (Anne Nagel) in the main part of the house. The audience learns that Cameron has bought an isolated house in a swampland area and that Lenora misses her boyfriend Tom Gregory (Johnny Downs), a newspaper reporter. Cameron says that he understands that but he remains in the house in order to finish his work. The following night, Cameron turns Pietro back into a wolf and sends him single to the swamp. The next scene is at a local inn where we see a little child playing with a ball. The child is seen requesting her mother to play for a few more minutes. The child is seen alone in her room who is completely unaware that Pietro is opening the window and approaching her. The child’s ball is seen rolling into the next room but he is not seen again. A minute later, her mother is heard screaming.

Cameron hears about the little girl’s fate the next day from the locals and now knows his formula works. He turns to his real priority which is destroying the scientists who ruined his career. Cameron can only do that by seeing to it that Pietro is alone with each scientist when he becomes a wolf. The rest of the film involves Cameron setting up elaborate scenarios to make that happen. The rest also involves Cameron finding out that he really can’t control Pietro in wolf form and that Pietro becomes wolf without his knowledge. Tom Gregory, Lenora’s newspaper man boyfriend, begins to suspect Cameron as the being behind the slayings.

Cameron’s first attempt at revenge is to take Pietro along with him when he visits Professor Blaine. He tells Pietro that he is being rewarded for his assistance by being taken to the city. Cameron tells Blaine that he will see proof of a scientific breakthrough if he receives himself and Pietro as guests. Once they are in Professor Blaine’s house, a prearranged phone call gives Cameron an excuse to leave Pietro alone with the Professor. Blaine agrees to give Pietro a second combined dose of the two injections if Cameron does not return at midnight. Though Cameron failed to do so and that incites Blaine to give Pietro the injection. Pietro becomes a wolf which is the ultimate end of Professor Blaine.

Professor Fitzgerald accepts Cameron’s invitation to come and to have a talk with him at his home. The two scientists quarrel and Professor Fitzgerald says “It’s not my fault, only the public gave your theories the ridicule they deserved.” The professor gets ready to leave the house but he, unwisely, agrees to give Pietro a ride. During the ride Pietro becomes wolf and in a panic Fitzgerald crashes the car. The unconscious scientist is found by the people of the town and by Tom Gregory, who are searching for the little girl’s killer. The Professor is taken back to Cameron’s house. Once Fitzgerald is there and all but the reporter has left, Cameron finishes him off with a lethal injection. By this time Pietro, still in his wolf form, has returned to the house.

While all this is happening, a thunder and lightning in conjunction with rain and storm begins and a bolt of lightning sets some of Cameron’s laboratory chemicals on fire. It is then that Lenora finds Pietro, still in wolf form, in another part of the laboratory. Pietro chases her and Tom all over the house. The couple escapes and Pietro turns on and kills Cameron instead.

After Pietro does that, the completely ignored fire brings the house down on both Pietro and Cameron. Tom goes back into the house just long enough to see that they are both dead. Lenora is left crying in the rain as the story ends.