Judith Fontaine (Regina Carrol) is looking for her sister Joanie, who has disappeared into the hippie community of Venice, California. It turns out Joanie has become the victim of Groton (Lon Chaney Jr.), an axe-wielding homicidal maniac working for Dr. Durray (J. Carrol Naish), who is really the last of the Frankensteins and is now running a house of horrors by the beach and is performing experiments on Gorton's victims. One night Count Dracula (Zandor Vorkov) visits the doctor, showing him the original Frankenstein creation that was buried in a nearby graveyard. The doctor revives it and uses it to take revenge on his professional rivals.

While Count Dracula raids the tomb of Frankenstein's Monster, hoping to resuscitate the beast, outside a beach carnival sideshow Joan Fontaine is beheaded by a ravaging man. Soon after, Joan's sister, singer Judith, initiates a search for the missing girl, beginning with Sgt. Martin at police headquarters. Despite Martin's advice to sit and wait and his warning that the carnival that Joan frequented harbors many white slavers, Judith visits the beach. Meanwhile, two hippie teenagers, Strange and Samantha, visit the sideshow, which is run by the crippled Dr. Duryea. One of the exhibits features the doctor's hulking, mute assistant, Groton, wearing a mask, but Duryea soon reveals that Groton is harmless. After the teens leave, Duryea and Groton retreat to the laboratory behind the exhibit to continue Duryea's secret experiments, in which he is attempting to harvest a serum that will cure all disabilities, including the one that has confined him to a wheelchair. To collect the serum, he habitually injects with Groton with a potion to make him vicious, sends him out to decapitate young women and then collects the women's blood, which has developed a special antibody in response to the extreme shock to their systems. He has recently reconnected Joan's head to her body and resuscitated her, planning to harvest her blood continually. After placing the sedated girl in a glass case, Duryea injects Groton and sends him out to the beach. Soon after, Duryea is visited by Dracula, who reveals that he knows Duryea's true identity: he is a scion of the Frankenstein family, discredited by the medical establishment years earlier after his real name was made public, by rival Dr. Beaumont. Planning to use Duryea's serum to make himself invincible, Dracula forces the scientist to comply by summoning flames from a ring on his finger to set the lab on fire. While Groton kills a young woman named Laura, Judith enters a nearby bar. When she inquires after one of Joan's boyfriends, a biker named Rico, the waiter secretly informs Rico, who drugs her coffee. Soon, a hallucinating Judith staggers through the bar, where she is rescued by Strange and Samantha. They bring her to the beachside apartment of Mike Howard, a local who recognizes Joan from Judith's photograph. After Judith awakens, Mike informs her that her sister fantasized about being a carnival freak. With the teens, they visit the sideshow in order to glean clues to Joan's disappearance, and inside show Joan's photograph to Duryea. Although he claims not to know her, they deduce that he is lying. Upon leaving, Samantha is harassed by Rico and his gang, who are stopped from beating Strange when Sgt. Martin appears. That night, while Mike and Judith enjoy a romantic beach walk, Duryea and Dracula work together to revive Frankenstein's Monster, and eventually succeed. Dracula takes the Monster to Dr. Beaumont's hospital, and there directs the Monster to kill the doctor. Meanwhile, Duryea is disturbed when Groton grows agitated and demands an injection, while Mike and Judith examine the underside of the pier on which the exhibit sits and find a trap door. Fearful, Judith insists they stop searching and the two sit on the sand nearby. Groton, transformed into a beast, wanders the beach and stumbles upon Samantha, who is being attacked by Rico and his gang. Groton kills the men and takes the unconscious Samantha back to the lab. Mike hears the sound of Groton closing the trap door and, upon investigating, finds Samantha's locket in the sand. Certain now that Duryea is connected to the girls' disappearances, Mike and Judith run into the exhibit and attempt to break into the lab. Duryea allows them to enter and then, as Judith stares in shock at her sister's body in the glass case, informs them they are his newest experiment "participants." Groton and Grazbo, the dwarf who collects tickets, circle Mike with axes, but when Grazbo accidentally falls on his blade, Judith flees, chased by Groton. Duryea shoots at Mike, who escapes into the exhibit. There, Duryea runs out of bullets, and when Mike traps him in the guillotine exhibit, the machine chops off the scientist's head. Meanwhile, Strange, searching for Samantha, has led Sgt. Martin to the sideshow, and when the police spot Groton chasing Judith along the beach, they shoot and kill him. However, Dracula has arrived, and now catches Judith and mesmerizes her. After tying her to the pier, he revives her, announcing that he plans to have the Monster attack her so he can collect her blood. Mike runs at them with a flare in his hand, using the fire to confuse the Monster, who attacks Dracula. While they struggle, Mike frees Judith and they flee, but Dracula turns his ring on Mike, setting him on fire. After Judith faints, Dracula, who once again has the Monster at his command, takes her to a church. There, the monster grows attracted to Judith, lashing out when Dracula tries to kill her. The Monster tears off Dracula's ring and chases him into the woods, where Dracula is able to rip off the Monster's limbs, killing it. Just then, however, the sun rises, and the count is not able to return to the church before the sun burns his skin. Exposed to the sun, he disintegrates just as Judith frees herself from her bonds. Outside, she finds Dracula's cape and ring, and remembers the horrors of the past night.