Teenager Trudy Morton (Sandra Knight), who lives with her uncle Carter Morgan (Felix Maurice Locher), has nightmares in which she dreams that she is a monster running about the streets at night. Trudy believes the dreams are real. Her boyfriend Johnny Bruder (John Ashley) does not, nor do her friends Suzie Lawler (Sally Todd) and Don (Harold Lloyd Jr.). Little does Trudy know, but she actually does turn into a monster at night, thanks to Carter's unpleasant lab assistant Oliver Frank (Donald Murphy). He lives with them, works in Carter's home lab and has been spiking Trudy's fruit punch with the formula that he and Carter are developing. Carter's goal is to eliminate all disease so that people can live forever; Oliver's goal is something quite different.
Carter's project stalls and he breaks into Rockwell Labs for Digenerol, the chemical he needs for his experiments. He does not know who Oliver actually is or that, with the help of Elsu the gardener (Wolfe Barzell), Oliver is secretly assembling what he calls a "perfect being", for Oliver Frank is actually Oliver Frankenstein, grandson of the original Dr. Frankenstein.
Police Lt. Boyd (John Zaremba) and Det. Bill Dillon (Robert Dix) investigate a report from a frightened woman (Charlotte Portney) that a female monster in a swimsuit attacked her. It is, of course, Trudy. They spot her and fire a few shots but miss. Oliver grabs her and drags her home to recover.
The next morning Carter asks Oliver if he has seen the newspaper story about a "Frankenstein monster" on the loose. Oliver scoffs at the story, but when Carter disparages the Frankensteins, Oliver literally jumps to his feet in their defense. Boyd and Dillon are visited by Mr. Rockwell (Voltaire Perkins) of Rockwell Labs. Rockwell says that the stolen Digenerol may be somehow related to the monster issue.
Back at the home lab, Elsu mistakenly enters through a secret door while Carter and Oliver are working. Oliver silently shoos him out and to distract Carter knocks the bottle of Digenerol from his hand, spilling every drop. Carter says that he must now steal more Digenerol.
Suzie visits Trudy, but they quarrel and as Suzi flounces off she makes a date with Oliver. The date goes badly, with Oliver attempting to force himself on Suzie. Since he needs a brain for his perfect being, he runs Suzie over with his car, killing her. Oliver will create a female perfect being, something the Frankensteins have never tried before. When Elsu asks why, Oliver says that "now we're aware the female mind is conditioned to a man's world. It therefore takes orders, where the other ones didn't." In awe, Elsu exclaims, "Frankenstein's daughter!" after which they always refer to it in the feminine.
While Oliver tries to reanimate "her", Boyd and Dillon arrive. As they tell Oliver that they suspect Carter is the Digenerol thief, "she" (Harry Wilson) comes to life, hideously scarred and looking not in the least like pretty, blonde Suzie. After the police leave, "she" escapes and kills a warehouse worker (Bill Coontz). Another worker, Mack (George Barrows), calls the cops.
At the house, Trudy and Oliver are talking when someone knocks on the front door. Trudy answers. It is the monster. Trudy screams and faints. Elsu coaxes "her" into the lab. When Johnny arrives and tells him what happened, Oliver convinces Johnny that Trudy has an overactive imagination.
Oliver wants the lab for himself and decides to kill Carter. But as he starts strangling him, Boyle and Dillon show up with more questions about the Digenerol. Oliver tells them that Carter stole it; Carter tells them that Oliver tried to kill him. Oliver persuades them that Carter is mentally ill, and they arrest Carter. Oliver then argues with Elsu, who refuses any further assistance, and Oliver has "her" kill Elsu. Afterwards, Oliver tells Trudy and Johnny that Carter has been arrested. When Johnny leaves for the police station, Trudy stays behind. Oliver reveals that his name is actually Frankenstein, not Frank, and shows her his creation again. Trudy faints once more, but awakens and goes to the police station herself. Boyle tells her and Johnny that Carter has died.
Boyle and Dillon return to the house to further question Oliver. When Boyle leaves, Dillon stays behind to keep an eye on Oliver. Dillon stumbles across "her" hiding place in the house and Oliver orders "her" to kill Dillon.
Trudy and Johnny come home and also find the monster's hideout. Oliver orders "her" to kill them, too. "She" and Johnny fight in the lab. Johnny throws a vial of acid at "her", but hits Oliver instead, melting his face. As Oliver falls screaming to the floor, the monster accidentally sets "herself" alight on a Bunsen burner. Trudy and Johnny flee as "she" is consumed by flames.
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