Dr. Gustav Niemann escapes from prison along with his hunchbacked assistant Daniel, for whom he promises to create a new, beautiful body. The two murder Professor Lampini, a traveling showman, and take over his horror exhibit. To exact revenge on Burgomaster Hussman, who had put him in prison, Niemann revives Count Dracula. Dracula seduces Hussmann's granddaughter-in-law Rita and kills Hussmann himself, but in a subsequent chase, Niemann disposes of Dracula's coffin, causing the vampire to perish in the sunlight. Niemann and Daniel move on to the flooded ruins of Castle Frankenstein in Visaria, where they find the bodies of Frankenstein's monster and Larry Talbot, the Wolf Man, preserved in the frozen castle. Niemann thaws them and promises to find Talbot a cure for the curse. Niemann is more interested in reviving the monster and exacting revenge on two traitorous former associates than in keeping his promises. Talbot transforms into a werewolf and kills a man, sending the villagers into a panic.
Niemann and Daniel save a gypsy girl named Ilonka, and Daniel falls in love with her; it is unrequited, however, as Ilonka falls in love with Talbot. Daniel tells Ilonka that Talbot is a werewolf, but she is undeterred, and promises Talbot that she will help him. Events reach a crisis point when Niemann revives the monster and Talbot again turns into a werewolf. The werewolf attacks and fatally wounds Illonka, but she manages to shoot and kill Talbot with a silver bullet before she dies. Daniel blames Niemann and turns on him. The monster intervenes, throws Daniel out of the window, and carries the half-conscious Niemann outside, where the villagers chase them into the marshes. There, both the monster and Niemann drown in quicksand.