Dr. Frederick Frankenstein is a lecturing physician at an American medical school and engaged to Elizabeth, a socialite. He becomes exasperated when anyone brings up the subject of his grandfather Victor Frankenstein, the infamous mad scientist, and insists that his surname is pronounced “Fronkonsteen”. When a solicitor informs him that he has inherited his family’s estate in Transylvania after the death of his great-grandfather, the Baron Beaufort von Frankenstein, Frederick travels to Europe to inspect the property. At the Transylvania train station, he is met by a hunchbacked, bug-eyed servant named Igor, and a young assistant, Inga. Upon hearing that the professor pronounces his name “Fronkensteen”, Igor insists that his name is pronounced “Eyegor”, rather than the traditional “Eegor”.
Upon arrival at the estate, Frederick meets Frau Blucher, the housekeeper. After discovering the secret entrance to his grandfather’s laboratory and reading his private journals, Frederick decides to resume his grandfather’s experiments in re-animating the dead. He and Igor steal the corpse of a recently executed criminal, and Frederick sets to work experimenting on the large corpse. Igor is sent to steal the brain of a deceased revered historian, Hans Delbrück; startled by his own reflection, he drops and ruins Delbrück’s brain. Taking a second brain labeled “Abnormal”, Igor returns with it, and Frederick unknowingly transplants it into the corpse.
Soon, Frederick is ready to re-animate his creature, who is eventually brought to life by electrical charges during a lightning storm. The creature takes its first steps, but, frightened by the sight of Igor lighting a match, he attacks Frederick and nearly strangles him before he is sedated. Meanwhile, unaware of the creature’s existence, the townspeople gather to discuss their unease at Frederick continuing his grandfather’s work. Inspector Kemp, a one-eyed police official with a prosthetic arm, whose German accent is so thick that even his own countrymen cannot understand him, proposes to visit the doctor, whereupon he demands assurance that Frankenstein will not create another monster. On returning to the lab, Frederick discovers Blucher setting the creature free. She reveals the monster’s love of violin music and her own romantic relationship with Frederick’s grandfather. However, the creature is enraged by sparks from a thrown switch and escapes the castle.
While roaming the countryside, the monster has encounters with a young girl and a blind hermit, references to 1931’s Frankenstein. Frederick recaptures the monster and locks the two of them in a room, where he calms the monster’s homicidal tendencies with flattery and fully acknowledges his own heritage, shouting out, “My name is Frankenstein!” At a theater full of illustrious guests, Frederick shows “The Creature”, dressed in top hat and tails, following simple commands. The demonstration continues with Frederick and the monster performing the musical number “Puttin’ On the Ritz”. However, the routine ends suddenly when a stage light explodes and frightens the monster, who becomes enraged and charges into the audience, where he is captured and chained by police. Back in the laboratory, Inga attempts to comfort Frederick and they wind up sleeping together on the suspended reanimation table.
The monster escapes when Frederick’s fiancée Elizabeth arrives unexpectedly for a visit, taking her captive as he flees. Elizabeth falls in love with the creature due to his “enormous schwanzstucker”. The townspeople hunt for the monster; to get the creature back, Frederick plays the violin to lure his creation back to the castle and recaptures him. Just as the Kemp-led mob storms the laboratory, Frankenstein transfers some of his stabilizing intellect to the creature who, as a result, is able to reason with and placate the mob. Elizabeth—with her hair styled after that of the female creature from the Bride of Frankenstein—marries the now erudite and sophisticated monster, while Inga, in bed with Frederick, asks what her new husband got in return during the transfer procedure. Frederick growls wordlessly and embraces Inga who, as Elizabeth did when abducted by the monster, sings the refrain “Ah, Sweet Mystery of Life”.