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In London, at the stroke of midnight, Lawrence Talbot calls Wilbur Grey, a bumbling American post office employee, and tells him to delay the delivery of two crates to McDougal's House of Horrors wax museum until he arrives.  After the call, however, McDougal appears and demands that Wilbur and his co-worker, Chick Young, immediately deliver the crates, which contain the bodies of Count Dracula and the Frankenstein Monster.

As Wilbur and Chick bring the crates to the creepy McDougal house, a lightning bolt causes the electricity to go out, and although Wilbur witnesses the vampire begin to rise, Chick does not believe his story.  Wilbur then opens the Monster's case and is terrified by the scarred face, at which point Dracula hypnotizes him long enough to escape with the Monster.  Just then, McDougal arrives, sees the empty crates and accuses Wilbur and Chick of robbing him.  As the men argue, Dracula spirits the Monster away to his island castle, where he and Sandra Mornay, the woman Wilbur believes to be his girlfriend, discuss their plans to implant Wilbur's malleable brain into the weakened Monster, whom they can then use to carry out their evil deeds.

Later, Talbot, who has rented a room at Wilbur and Chick's apartment house, informs the two men that, to his horror, he is the Wolfman, and that they must help him stop Dracula from carrying out his evil plan.  He then convinces them to lock him in his room for the evening, so that when he turns into a werewolf, he cannot harm anyone.

The next day, Joan Raymond, the beautiful insurance invvestigator hired by McDougal, visits Wilbur and flirts with him as part of a plan to find out where he has hidden McDougal's stolen displays.  She convinces him to take her out that night, although he already has a date to take Sandra to a costume ball.  That night, when Chick brings Wilbur and Joan to meet Sandra at the castle, Joan flirts with laboratory assistant Dr. Stevens.  While they wait for Sandra to put on her costume, Talbot calls Wilbur at the castle to warn him that Dracula and the Monster are in the building.  Wilbur tells Chick, who insists that they search the basement to prove that nothing is there, and although Wilbur finds the ghouls, each time the ghouls chase him, Chick is out of the room and does not see them.

Upstairs, Sandra discovers from Joan's identification card that she is an investigator who might uncover their plan and so runs downstairs to dissuade Dracula, now disguised as a doctor, from stealing Wilbur's brain that night.  Dracula, who does not want to postpone his scheme, bites Sandra, and she becomes a vampire under his spell.

Later, at the ball, Sandra tries to lure Wilbur back to the castle, while Dracula hypnotizes Joan.  At the same time, Talbot transforms into the Wolfman and attacks McDougal, who assumes that Chick, who is wearing a wolf costume, is the culprit.  Chick and Wilbur run from the police into the woods, and when Joan follows, Dracula abducts Wilbur and Joan as Chick faints from fear.  When he wakes, Chick finds the now-human Talbot and brings him to the castle, just as Sandra prepares the operation to transplant Wilbur's brain into the Monster.  Chick and Talbot burst in just in time to save Wilbur, but soon the moon rises, turning Talbot into a werewolf again.  The Monster, meanwhile, frees himself from his bonds, grabs Sandra and throws her out the window.  Chick and Wilbur run from the Monster while the werewolf pursues Dracula into the ocean, where they both drown.  Joan wakes from her hypnosis and helps Dr. Stevens set the Monster on fire, allowing Chick and Wilbur to escape by boat.  They set sail, not realizing until they hear a disembodied voice that the Invisible Man is riding with them.