It seems that Count Dracula (Bela Lugosi), in league with a beautiful but diabolical lady scientist (Lenore Aubert), needs a simple, pliable brain with which to reactivate Frankenstein's creature (Glenn Strange). The ideal brain belongs to the hapless Lou Costello, whom the lady doctor woos to gain his confidence and lure him to the operating table.
Lawrence Talbot (Lon Chaney Jr.), better known as the Wolf Man, arrives on the scene to warn Costello and his pal Bud Abbott of Dracula's nefarious schemes.
Throughout the film, the timorous Costello witnesses the nocturnal rituals of Dracula and the Monster, but can't convince the ever-doubting Abbott--until the wild climax in Dracula's castle, where the comedians are pursued by all three of the film's monstrosities.
As a bonus, the Invisible Man (voiced by an unbilled Vincent Price) shows up for all the excitement.