Dr. Frankenstein dares to tamper with life and death by creating a human monster out of lifeless body parts.
Mary Shelley reveals the main characters of her novel survived: Dr. Frankenstein, goaded by an even madder scientist, builds his monster a mate.
Baron Wolf von Frankenstein (Basil Rathbone), son of Victor Frankenstein, relocates his wife Elsa (Josephine Hutchinson) and their young son Peter (Donnie Dunagan) to the family castle. Wolf wants to redeem his father's reputation, but finds that such a feat will be harder than he thought after he encounters hostility from the villagers, who resent him for the destruction his father's Frankenstein's Monster wreaked years before. Aside from his family, Wolf's only friend is the local policeman Inspector Krogh (Lionel Atwill) who bears an artificial arm, his real arm having been "ripped out by the roots" in an encounter with the Monster as a child.
The residents of the village of Frankenstein feel they are under a curse and blame all their troubles on Frankenstein's monster. The Mayor allows them to destroy Frankenstein's castle. Ygor finds the monster released from his sulfuric tomb by the explosions. The exposure to the sulfur weakened yet preserved the monster. Ygor and the monster flee the castle, and the monster is struck by a bolt of lightning. Ygor decides to find Ludwig, the second son of Henry Frankenstein, to help the monster regain his strength.
Dr. Gustav Niemann (Boris Karloff) escapes from prison along with his hunchbacked assistant Daniel (J. Carrol Naish), for whom he promises to create a new, beautiful body. The two murder Professor Lampini (George Zucco), a traveling showman, and take over his horror exhibit. To exact revenge on Bürgermeister Hussman (Sig Ruman), who had put him in prison, Niemann revives Count Dracula (John Carradine). Dracula seduces Hussmann's granddaughter-in-law Rita (Anne Gwynne) and kills Hussmann himself, but in a subsequent chase, Niemann disposes of Dracula's coffin, causing the vampire to perish in the sunlight.