Wheelchair-bound mad scientist Dr. Durea (J. Carrol Naish), the last descendant of the original Dr. Frankenstein, takes to murdering young girls for experimentation in hopes of perfecting a blood serum of his own creation with help from his mute, simple-minded assistant Groton (Lon Chaney, Jr.). Count Dracula (Roger Engel (co-producer Samuel M. Sherman’s stockbroker) listed in the film’s credits under the pseudonym “Zandor Vorkov”) comes to the scientist, promising to help him revive the Frankenstein Monster (which he has exhumed from its secret grave in nearby Oakmoor Cemetery) in return for Durea’s serum, which he hopes will grant him the ability to go out in the sunlight, thus making him invincible.
As a cover, the duo work out of a secret laboratory hidden behind the Creature Emporium, a haunted house exhibit and a throwback to the old sideshow days located on the boardwalk amusement park in Venice, California. They bring the Monster (John Bloom) back to life and Durea sends him and Dracula out to exact revenge on the man who both discredited him and crippled him in a laboratory fire, Dr. Beaumont (Forrest J Ackerman). Las Vegas showgirl Judith Fontaine has also previously arrived, looking for her missing sister Joanie (Maria Lease) who was last seen hanging out with a group of hippies led by Strange (Greydon Clark). Judith has gotten no satisfaction from Sgt. Martin (Jim Davis). She says she is going to investigate on her own and does so, attracting the attention of biker Rico (Russ Tamblyn) and his gang. Rico slips her some LSD at a dive bar with the bartender’s help and Judith, while on a trip due to the drug, is taken by Strange and his girlfriend Samantha (Anne Morrell), who have just finished attending a protest, to the home of aging fellow hippie Mike Howard (Anthony Eisley) who agrees to help her find Joanie. Judith, Mike, Samantha and Strange go to the Creature Emporium (which Joanie had been known to go to many times) and Judith shows Durea a picture of Joanie, asking her if he has seen her, but he says that he has not.
More girls turn up missing, the Monster kills a couple of police officers while trying to kill a girl for Durea’s experiments and Groton takes to the beach with an ax and kills Rico and his gang who were attacking Samantha, then Groton takes her into Durea’s laboratory through a trap door with a ladder that leads to the beach below the Creature Emporium. Judith and Mike go to the Emporium, discover the trap door and the laboratory and confront Durea, who explains that the girls who were killed (including Joanie, whom Judith finds preserved naked, unmoving and seemingly neither dead or alive, in a glass-fronted box (with strategically-placed wide black lines on the glass that are over her breasts and vagina) in the laboratory, along with Samantha who is in another identical box, in a similar state of nakedness and immobility and also seemingly neither dead or alive) were frightened before their deaths and this created a special enzyme in their blood, which is the main ingredient for his blood serum. He also tells Judith that, after he has Mike (with whom she has fallen in love and he with her) killed, her fear upon seeing Mike’s death will help him complete the serum at last (it is implied, therefore, that she will be killed as well). Durea sends Groton and a dwarf named Grazbo (Angelo Rossitto), the ticket taker at the Creature Emporium, after the couple (Durea’s original reason for creating the blood serum in the first place was to heal his crippled legs and to transform Groton and Grazbo into normal people). Grazbo falls through the laboratory’s trap door to the beach (which was accidentally opened) and onto an ax that he had dropped beforehand while holding on to the ladder, which kills him, and Groton goes after Judith. Sgt. Martin and Strange arrive with the police and Martin shoots Groton from the rooftop of the building from which he falls to his death, while Durea falls from his wheelchair into a guillotine display in the Emporium while attempting to escape and is beheaded in it.
Dracula meets Judith, hypnotizes her and binds her with rope to a railing. He then confronts Mike, who shoves a lit car flare in the Monster’s face, forcing him to briefly turn on Dracula in his pain. Mike unties Judith and they run away, but as they do so, Dracula blasts Mike with fire shot out from his one-eyed demon-headed ring, burning him to ashes.
Judith faints upon seeing Mike’s death and slowly awakens to find herself bound with rope again to a chair in an abandoned and desecrated church in a forest area outside of Venice where Dracula’s coffin is hidden. Dracula is about to drink her blood and turn her into his vampire bride, but the Monster (who has fallen for her beauty) turns against Dracula and forces him out of the church and into the forest outside it (but not before removing Dracula’s ring from his finger before exiting the church), where a fierce battle ensues between the two monsters. Dracula literally tears off the Monster’s arms and head, but gets caught in the rays of the sun before he can make it back to his coffin and slowly disintegrates into dust. Judith manages to free herself and sees Dracula’s ashes and clothing. She then picks up Dracula’s ring at the door of the church, but, after a brief flashback of all that has happened to her before, drops it and flees in fear.