The Bride!

Speaking from the afterlife, Mary Shelley says she has a story she wanted to tell after Frankenstein, but could not due to her death. To tell it, she possesses Ida, a woman living in 1936 Chicago, who in her trance proceeds to discuss the criminal activities of crime boss Lupino. Lupino's henchmen Clyde and James discreetly take her aside where Shelley's ranting causes her to fall down a flight of stairs to her death.

Elsewhere in Chicago, Frankenstein's monster, AKA "Frank", arrives at the house of scientist Dr. Cornelia Euphronius. Having read about Euphronius' work on reanimation, he enlists her to create a companion for him after a century of loneliness. Euphronius and Frank choose Ida's corpse and successfully revive her, but she loses her memory in the process. Frank takes advantage of this and states that she is his bride and lost her memory in an accident.

Frank and Ida see a movie featuring Frank's favorite actor Ronnie Reed and then go dancing at a club. As they leave, two men attempt to assault her and Frank kills them. Frank tells her to leave him, but she decides to run away with him. They stow away in a train to New York City, killing a security officer who discovers them after Shelley causes her to have an outburst. Meanwhile, Detective Jake Wiles and his assistant Myrna Malloy investigate the murders and eventually follow them to New York.

While the pair are in hiding, Frank tells her her name is Penelope Rogers. Penelope causes chaos in a screening of Revolt of the Zombies and they seek refuge from the police and an angry mob at a high-class party. Frank runs into Ronnie Reed and expresses his adoration, but is dismissed. Frank begins to dance like Reed does in his films, Penelope joining him, as do party-goers possessed by Mary Shelley. Detective Wiles and the police arrive at the party, but fall back when Penelope holds Reed at gunpoint and calls out men in attendance for abusing women. Wiles recognizes her real identity as Ida. Penelope kills a police officer, and she and Frank escape the city in a stolen car.

Penelope and Frank's crime spree becomes well-publicized and Lupino recognizes her in a newspaper. He executes James for failing to silence her and dispatches Clyde to kill her again. Meanwhile, Malloy makes a connection between reports of murders/sightings of the pair in cities where Reed's films were set. This prompts Malloy and Wiles to travel to Niagara Falls where they find the couple. Wiles confronts them and informs Penelope of her previous identity as Ida before she shoots him in the foot. Fleeing, an angered Penelope takes the injured Frank to a drive-in, where he tells her the truth of her origins.

Wiles explains to Malloy that he and Ida were planning to take down Lupino and expresses guilt that his inaction led to her death. He decides to retire and promotes Malloy as his replacement. Malloy tracks the couple to the drive-in. Frank apologizes for his actions and affirms his love for Penelope/Ida who decides to call herself the Bride. As he proposes, Clyde fatally shoots him but the Bride escapes with Frank's body, returning to Euphronius' laboratory, followed by Malloy.

Euphronius refuses to reanimate Frank, citing that her attempts to reanimate her husband went wrong, leading her to have to kill him. Clyde breaks into the laboratory and fatally shoots the Bride before fleeing. The police arrive before Malloy orders them to search for Clyde, leaving Euphronius with the Bride's and Frank's corpses. Encouraged by Malloy and Mary from the afterlife, Euphronius decides to revive the couple. Outside, the police arrest Clyde. Malloy sees bright lights shine from the laboratory. Inside, the revived corpses hold hands.

During the credits, Lupino is apprehended by Wiles and female rioters inspired by the Bride.