Boris Karloff plays Dr. Bernard Adler, a small-town physician developing a serum to cure a young girl's polio using spinal fluid, who resorts to murdering local townspeople and disguising himself in the skin of an escaped circus ape to cover his tracks and harvest what he needs. Directed by William Nigh, it's one of several "mad scientist" B-pictures Karloff made for Monogram in the early 1940s, trading his more iconic monster roles for a more overtly villainous, morally compromised character.