In 1860, Baron Frankenstein (Peter Cushing) escapes execution by having a priest guillotined in his place, with the help of his hunchbacked assistant Karl (Oscar Quitack). Three years later, calling himself Doctor Stein, he is a successful surgeon in Carlsbruck, catering to the wealthy but also treating the poor in a pauper’s hospital. He is recognized by Doctor Hans Kleve (Francis Matthews) who offers to help him with his experiments. Karl offers his brain, wanting a new body now that a lovely new assistant named Margaret (Eunice Grayson) is working at the hospital.

The transplant is a success, but Karl panics and talks Margaret into freeing him. He is worried because a chimp into which Victor transplanted the brain of an orangutan has become violent. Karl runs from the hospital and hides in Victor’s laboratory, where he burns his old hunchbacked body. A drunken janitor thinks he’s a burglar and attacks him, bur Karl kills him. Victor and Hans search for him.

The next morning, Margaret finds Karl in the stables. When she returns with Hans, Karl is gone. That night, he strangles a local girl. The next night, his deformities are back and he breaks into a reception, addressing Victor by his real name before dying. Victor denies being the infamous Doctor Frankenstein, but his grave is opened and the body of the priest is found. The patients at the hospital, realizing that Frankenstein has been experimenting on them, attack Victor and Hans rushes him to the laboratory. The police arrive, but Hans shows them Victor’s body. Then he transplants Victor’s brain into a new body. Later, in London, Hans is assistant to a certain Doctor Franck, and they open a practice.

The film was directed by Terence Fisher for Hammer Film Productions as a sequel to Curse of Frankenstein. When producer James Carreras finished selling the unmade film in America, scriptwriter Jimmy Sangster pointed out that they had killed off Frankenstein, and Carreras said, “You’ll think of something.”  It received praise from critics as a high-grade horror film and still has 92% approval on Rotten Tomatoes. The Daily Telegraph was horrified by the gore and suggested a special category of film called For Sadists Only. Every morning, the chimpanzee gave Peter Cushing a big kiss. Hammer Studio executives said that a shot involving the dropping of Karl’s brain into a jar had been removed by demand, but the scene has appeared in every version of the film.