Released 13 January 1939, Son of Frankenstein marked the final time Boris Karloff played the Monster, the first time Béla Lugosi played Ygor, and the last A production in the Frankenstein franchise. It was a huge shot in the arm to Universal’s declining horror reputation.
On 5 April 1938, an almost-bankrupt L.A. theatre screened Frankenstein, Dracula, and King Kong. It was a major moneymaker and inspired many other successful revivals. Universal, seeing dollar signs, decided to make another Frankenstein sequel.
James Whale, director of Frankenstein and Bride of Frankenstein, didn’t want to do another horror film. In his place, Universal chose Rowland V. Lee.
Baron Wolf von Frankenstein (Basil Rathbone), Dr. Henry Frankenstein’s son, moves his wife Elsa (Josephine Hutchinson) and their little boy Peter (Donnie Donagan, now 85 years old) to the family castle upon coming into his inheritance.
Wolf’s enthusiasm for this new chapter of his life isn’t shared by his family, nor anyone else. The house gives Elsa and Peter the creeps, and the locals deeply resent their existence. After all, Wolf’s dad created a monster who terrorized them.
Inspector Krogh (Lionel Atwill) visits on the first night to try to warn Wolf away. Krogh’s right arm was torn from the roots by the Monster when he was a boy, something he’s never forgotten. He tells Wolf the Monster may still be at large, committing murders, despite being believed dead for years.
Across from the castle is Henry’s old lab, whose roof was blown off when the Monster was destroyed. Wolf eagerly goes to explore it after breakfast, and encounters Ygor. Earlier, Ygor peered in on Peter while he was sleeping.
Ygor is a body-stealing blacksmith who survived a hanging and now lives in the old lab, away from the eyes of the world. His neck was permanently deformed by the hanging.
Ygor takes Wolf to the family crypt, where his grandfather and father are entombed. Also in the crypt is the Monster’s comatose body.
Ygor says they’re friends, and that the Monster does things for him. The Monster is now comatose because he was struck by lightning under a tree while hunting. He can’t die because Henry made him live for always.
Ygor demands Wolf reanimate the Monster, on condition he not be seen by anyone.
With help from Ygor, Wolf hauls the Monster’s body up into the lab and tethers him to the table he was brought to life upon. Ygor pushes Wolf’s loyal assistant Benson (Edgar Norton) out of the door, but ultimately relents when Wolf explains how valuable Benson is.
Wolf and Benson meticulously examine the Monster every which way to determine what kind of state he’s in. Startling discoveries are two bullets in the lung and very unusual blood.
Ygor is hauled before the court to spill all he knows about Wolf and his experiments. If he doesn’t cooperate, he’ll be hanged again, properly this time. Ygor argues he was legally pronounced dead, and is told to leave and not cause trouble.
After concluding his extensive examinations, Wolf says as a human he should destroy the Monster, but as a scientist, it’s his duty to reanimate his father’s creation.
After Benson turns on the generator, the process initially seems to work very quickly. However, the signs of life fade away, appearing mere reflexes. Wolf declares the Monster is too comatose to reanimate.
While dining with Krogh, it comes out that Henry’s lab was built by the Romans, over a natural sulphur pit used as mineral baths. The sulphur is now over 800 degrees. Krogh doesn’t know how Wolf can bear to work with those sulphur fumes.
Peter’s innocent babble also reveals the Monster indeed reanimated and is on the loose. Full of a foretaste of horror, Wolf rushes off to the lab.
Ygor is nowhere to be found when Wolf arrives, but Wolf does find the Monster. Differing from the previous two Frankenstein films, he now wears a fur vest and can no longer talk.
When Ygor arrives, Wolf insists the Monster can’t leave. No one can know he’s there, despite Ygor’s claim the Monster only does what he tells him. Wolf also says he must continue his experiments. The Monster can walk, but his mind isn’t well yet.
Back at the castle, Wolf tells Benson what happened and swears him to secrecy. Despite his sheer terror, Wolf is determined to finish his work and become the greatest scientist of all time. He trusts the Monster will only do what Ygor bids him.
Trouble begins when Benson disappears. Ygor reports he ran away in fear of the Monster, but Wolf is terrified the worst happened.
And thus begins a new wave of horror as the Monster prowls through the town and the villagers seek blood revenge on Wolf.