Frankenstein and his Monster revive, and another mad scientist collaborates to make a mate for the Monster, now learning to talk and becoming more human.
In a prolog the poet Byron relishes the horrors of Mary Shelley's Frankenstein novel, but Mary (Elsa Lanchester) considers it a moral lesson to men who would imitate God. She agrees to continue the story.
People watch the fire that burned the Monster. Hans looks for the body. In a stream the Monster (Boris Karloff) kills Hans and his wife. Minnie (Una O'Connor) says it is alive, but she is not believed. Henry Frankenstein (Colin Clive) revives, saying he is cursed for giving life like God. Elizabeth (Valerie Hobson) calls it blasphemy and becomes hysterical. Dr. Pretorius (Ernest Thesiger) comes to see Frankenstein at night and asks to work together. Frankenstein goes with him to his home. Pretorius drinks and shows him miniature people he grew from seed. He says they will create a woman as a mate to make a new race.
The Monster finds a woman and is wounded by a hunter. People are aroused and track down the Monster, binding him to a log and putting him in a dungeon chained; but he breaks out. In the woods he comes upon campers who scatter. The Monster hears the violin of a blind hermit (O. P. Heggie), who welcomes him. The Monster shows he understands. The hermit gives him food, tells him to sleep, and prays. He teaches the Monster to speak and smoke, though the Monster fears fire. When two travelers arrive and see the Monster, he leaves and goes to a cemetery, as men search for him. He sees Pretorius and two hired men enter the crypt. Pretorius stays and drinks. The Monster talks, eats and drinks; he is to have a woman friend.
Pretorius calls on Frankenstein and his wife Elizabeth. He has a living brain. The Monster comes in and tells Frankenstein to sit down. Pretorius tells the Monster to leave, and he does so, taking Elizabeth. Pretorius promises she will be returned if Frankenstein agrees to experiment. Frankenstein gets a heart beating, but it stops. They send Karl for a newly dead female, promising him 1,000 crowns. Frankenstein gets the heart going but is tired. Pretorius gives the Monster a drink so that he passes out. Pretorius lets Frankenstein talk on a telephone to Elizabeth. They get the body ready for an electric shock from lightning by kites. The Monster throws a man off the tower. The woman comes alive, smiles at the Monster and screams. Frankenstein goes out with Elizabeth. The Monster blows up the tower with himself, his bride, and Pretorius.
This powerful parable warns of blundering human experimentation and reflects the nightmarish Nazi aim to create a master race by violent means.