Few years have passed after the brawl between the Frankenstein's Monster and the Wolf Man. The film opens with Mad Scientist Dr. Niemann (Boris Karloff) escaping a prison, where he was sent after experimenting with the dead, with a hunchbacked murderer Daniel. After taking over a travelling "Chamber of Horrors", Niemann sets out to do the first thing on his mind: Revenge upon those who sent him away for fifteen years.
He finds that the main event of his newly acquitted sideshow, Dracula's (John Carradine) staked corpse, is pretty much real and sends him after his first target in the town of Riegelberg. After that business, Niemann sets out to village of Frankenstein (previously known as Vasaria) and finds the bodies of Larry Talbot (Lon Chaney Jr.) and the Monster (Glenn Strange) encased in ice under the remains of Ludwig Frankenstein's sanitarium. He decides to revive the Monster and promises to help Lawrence Talbot on breaking his werewolf curse to get his assistance. Then they set out to Niemann's old place in Visaria (not to be confused with the aforementioned Vasaria) to begin the experimenting...
More Monster Mash would eventually be provided by the next sequel, House of Dracula. In 1997, NBC created a mini-series remake of the movie with the backing of Universal. See House of Frankenstein (1997) for more information.
Upon entering the House of Frankenstein, you'll find:
- Arc Words: A certain poem is repeated one and half times. You know the one.
- Breaking the Bonds: The Monster, upon seeing the man who brought him back to full power being almost strangled to death, breaks free from the operating table that he was strapped to for vengeance against the perpetrator.
- Chase Scene: After Dracula kidnaps Rita and races away on his carriage, her husband and the police chase after him on horseback.
- Chronic Backstabbing Disorder: It's probably easier to list the characters Niemann doesn't betray (or at least plan to).
- Crossover: Upping the ante from the previous film, Dracula is now added to the group, retroactively expanding a shared universe that now included Dracula (1931), Frankenstein (1931), Dracula's Daughter, Son of Dracula, Bride of Frankenstein, Son of Frankenstein, The Ghost of Frankenstein, The Wolf Man (1941) and Frankenstein Meets the Wolf Man.
- Dead Guy on Display: Lampini stole Dracula's staked remains from his castle at the Carpathians, and now has him as his main attraction in his "Chamber of Horrors".
- Dead Person Impersonation: Dr. Niemann has Daniel kill Lampini, and takes his place as the owner of "Lampini's Chamber of Horrors".
- Destination Defenestration: The Monster throws Daniel, his tormentor and the attempted murderer of his reviver Dr. Niemann, out of a window to his doom.
- Dramatic Thunder: When the guard the Prison holding Dr. Niemann tells him that he'll never get out, he answers that he will, which is then punctuated by a flash of lightning combined with thunder.
- For Science!: No other motivation for Dr. Niemann's attempt at trying to place a human brain on a dog is given beyond this.
- Harmless Freezing: Both the Monster and the Wolf Man are found enclosed in ice, but only Wolf Man is unharmed by it. The Monster has been damaged by freezing and its restoration is one of Dr. Niemann's goals.
- Hostile Weather: The storm in the beginning of the film, which eventually helps Dr. Niemann and Daniel to escape from prison thanks to a convenient lightning strike breaking its walls.
- Hot Gypsy Woman: Ilonka. Daniel's certainly smitten.
- Human-to-Werewolf Footprints: Talbot's first onscreen werewolf transformation is shown through this.
- The Igor: Daniel, a hunchback murderer who joins Dr. Niemann in hopes that he''ll give him a better body someday.
- Love Triangle: Daniel loves Ilonka, Ilonka loves Larry and Larry only wants to die.
- Mind-Control Device: Dracula's ring, which when placed on Rita turns her into a willing would-be victim for him.
- Mutual Kill: Ilonka finally works up the courage to shoot the Wolf Man with a silver bullet (powered by true love, no less), but the Wolf Man fatally mauls her in the process, and she dies embracing Larry's body.
- No Immortal Inertia: After Dracula is exposed to the sunlight, he turns into a skeleton that he started out as in the film.
- Our Vampires Are Different: Dracula can apparently only feed in bat form, and is revived after the stake is removed from his skeleton.
- Pietà Plagiarism: Daniel carries Ilonka's corpse back to Dr. Niemann's laboratory after she is killed by Talbot in this manner.
- Quicksand Sucks: The angry mob searching Dr. Niemann's property purposedly drive the Monster carrying him into a bog, which then swallows them both.
- Rampage from a Nail: Inverted. Removing the stake from the heart area of Dracula's skeleton lets him instantly re-form and go on a "rampage" of bloodsucking.
- Shadow Discretion Shot: Dracula's transformation into a bat and subsequent bloodsucking from Burgomaster Hussman is shown through shadows.
- Shout-Out: An athletic hunchbacked man falling for a Romani woman? Why does that sound familiar? Not only that, but the Romani woman's corpse is entangled with the body of an unrequited lover (although in this case, it's unrequited on her end, rather than his). Even Daniel, jealous as he is, seems reluctant to separate them.
- Silver Bullet: After having her fears confirmed, Ilonka makes one to release Larry from his curse.
- A Taste of the Lash: Daniel takes his frustrations on the weakened Monster, who is currently Strapped to an Operating Table, with a belt. Which has been proven to be such a good idea in the past.
- Torches and Pitchforks: A mob is organized in Visaria after Larry's werewolf killings. They eventually find their way to Dr. Niemann's place for a climax.
- Trailers Always Lie: We were promised that this Monster Mash would have Dracula, Wolf Man, and Frankenstein's Monster all together. While they're all in the movie, Drac dies before Frank and Wolfy even show up, and the only scenes shared by the latter two are while Frank's in a coma and Wolfy is in his human form.
- Transformation Sequence: Talbot's final transformation into a werewolf on front of a mirror, shown through the traditional dissolving cuts.
- Wacky Wayside Tribe: The entire subplot involving Dracula, all of which happens under twenty minutes and has no bearing (or is even mentioned) on rest of the film after it is finished.
- The X of Y: The film's title.
- Yank the Dog's Chain: The Mad Scientist villain keeps promising to cure Talbot, but has no intention of doing so and both Talbot and his love interest end up dead because of it.