The film focuses on the exploits of the vengeful Dr. Gustav Niemann (Boris Karloff), who escapes from prison. He is helped by the hunchback 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 BÃ¼rgermeister Hussman (Sig Ruman), who had once caused his imprisonment, 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 sunlight.
Niemann and Daniel move on to the flooded ruins of Castle Frankenstein, where they find the bodies of the Frankenstein Creature (Glenn Strange) and Lawrence Talbot the Wolf Man (Lon Chaney, Jr.) preserved in the frozen waters. Nieman thaws out the two and promises to find Talbot a cure for the curse. However, he is more interested in reviving the Creature and exacting revenge on two former associates than in keeping his promises. Talbot transforms into a werewolf and kills a man, arousing the villagers.
Talbot is also envied by the hunchback Daniel, as both love Ilonka (Elena Verdugo), a gypsy girl. She has fallen in love with Talbot. Daniel reveals Talbot's curse to Ilonka but she is not deterred and promises to help him fight the curse.
Events reach a crisis point when Niemann revives the Frankenstein monster and Talbot again turns into a werewolf. Ilonka shoots Talbot with a silver bullet, thereby releasing him, but passes out on top of Talbot afterwards. Daniel blames Niemann and begins to strangle him. The Creature intervenes, throws Daniel out of the window, and carries the half-conscious Niemann outside, where the villagers begin to chase them and drive them into the marshes. There, both the Creature and Niemann drown in quicksand.
Who is the hunchback?
Daniel