Because this is really Lugosi's film.
Lugosi plays a surgeon named Dr. Vollin, who falls in love with a dancer whose life he saved. Which would normally be fine, except that the dancer is engaged to someone else and the Doctor is crazier than bat guano. His other hobbies include Edgar Allen Poe and building torture devices in his basement, so you can really see where this is going.
Not surprisingly, when he doesn't get his way on the girl, he decides the best solution is to invite everyone over to his house for a night of murder and torture.
Karloff has kind of a minor role as a criminal who becomes the unwitting dupe of Lugosi's mad doctor. It was really interesting because at this point in their careers, Karloff was the bigger star (hence, why he got top billing.) Karloff would state later that Lugosi boxed himself in by never learning to speak English proficiently, and that was probably true to a degree, but this film along with the Black Cat showed he had some range as an actor.
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