|“||You think I'd wreck the work of a lifetime because you're in love with a, a Gypsy girl?||„|
|― Doctor Niemann|
Gustav Niemann was a scientist who was greatly inspired by the works of Doctor Henry Frankenstein. He had learned about Frankenstein's experiments with reanimating dead tissue through his brother Fritz, who worked as Frankenstein's assistant.
In the village of Visaria, Niemann attempted his own experiments with cell reanimation and brain transplants. Like Frankenstein, Niemann robbed graves and collected the bodies of the dead in an effort to transfer the brain of a human being into the body of a dog. When the locals learned of Niemann's work, he was arrested. During his trial, his own assistant, Ullman, testified against him. Another witness, Frederick Strauss, testified that he saw Niemann robbing the graves of the newly dead. Following his trial, Gustav was sent to Neustadt Prison where he spent fifteen years in jail.
In prison, he befriended a cell mate named Daniel. Daniel was a hunchback, but Niemann was confident that his scientific powers could cure him should he ever get the opportunity. In exchange for his servitude, Niemann promised to transplant Daniel's mind into a perfect body.
One evening, terrible thunderstorms raged in the skies overtop the prison. A lightning bolt struck the side of the prison, causing an entire section of the building to collapse. Niemann and Daniel took this opportunity to escape.
They caught a ride with a traveling showman named Bruno Lampini and his caravan, "Lampini's Chamber of Horrors". Without revealing his name, Niemann told Lampini that Daniel and he were merchants held for ransom by mountain bandits for three months. Lampini boasted about possessing the remains of the vampire lord, Count Dracula, which he now displayed as the main attraction of his Chamber of Horrors, but Niemann was incredulous. He asked Lampini about a man named Hussman, one of those responsible for sending him to prison. Lampini told him that Hussman was now the Burgomeister of the village of Reigelberg. He ordered Lampini to take them to Reigelberg, but Lampini refused, so Niemann had Daniel strangle him. From there, Daniel and he cleaned themselves up and Niemann assumed Lampini's identity and traveled on to Reigelberg.
At the crossroads in Reigelberg, Niemann played the part of showman and presented Dracula's coffin to passing onlookers. Herr Hussman and his grandson Carl, along with Carl's wife, Rita, attended the showing, but Hussman didn't immediately recognize Niemann. He assured the Burgomeister that he would remember him in due time.
After the show, Niemann opened up Dracula's coffin and withdrew the wooden stake from the skeleton that resided within it. Dracula reformed and tried to use his powers to hypnotize Niemann, but his will was too strong. Holding the stake over Dracula's heart, Niemann made a deal with the vampire. In exchange for Dracula’s service, Niemann would agree to guard the vampire's coffin during the daylight hours. Dracula agreed to the arrangement. Niemann and Dracula's relationship was cut short soon after as Dracula was destroyed after being exposed to the sun.
Niemann and Daniel then traveled to the village of Frankenstein. It was his hope that by inspecting the ruins of Castle Frankenstein, he might be able to recover some of Henry Frankenstein's lost records. From there, Niemann would be able to continue his experiments. Niemann and Daniel eventually made their way into the castle and discovered a glacial cave beneath the ruins. Frozen in the ice beneath the castle were the remains of the Frankenstein Monster and the Wolf Man.
Niemann thawed them out and the Wolf Man turned back into his human form of Larry Talbot. Niemann promised Larry that if he helped him to find the Frankenstein records, then he would use his scientific knowledge to put Talbot’s brain into a new body, freeing him of the curse of the werewolf. Talbot agreed and quickly recovered Frankenstein's records.
Niemann then had the crew (which by this point included a gypsy girl named Ilonka) relocate to his laboratory in Visaria. Although he intended on helping Talbot, he was more concerned about his own research, not to mention exacting revenge against the men who sent him to prison. When they reached the laboratory, Niemann put Daniel and Talbot to work, fixing up the equipment and preparing for the operation. Talbot knew that time was of the essence for the moon would soon rise, and he would become the Wolf Man once again. Niemann told Talbot to be patient and that it would soon be time to perform the experiment.
That evening, Doctor Niemann and Daniel tracked down the last two men who testified against him at his trial: Ullman and Frederick Strauss. Abducting them and bringing him into his caravan, he told them how he was going to place each of their brains into the bodies of the Frankenstein Monster and the Wolf Man. He brought them back to his laboratory and prepared them for the operation, but his work was interrupted once again. Talbot insisted on Niemann performing the operation that would save his life immediately, but Niemann needed more time to conduct research on the Frankenstein Monster. He never told Larry his true intentions.
When the moon rose, Talbot turned into the Wolf Man for the second night in a row. During this time, he attacked Ilonka who shot him with a silver bullet, but in so doing, lost her own life. Daniel, who had been deeply infatuated with Ilonka, blamed Niemann for her death. If he had kept his promises to both himself and Talbot, Ilonka might have lived. Daniel betrayed Niemann, lunging at him in an effort to strangle him. As the two men grappled, the Frankenstein Monster awoke. He grabbed Daniel and hurled him through the laboratory skylight to his death. He then grabbed the injured Niemann and tried to leave the castle.
A horde of villagers discovered them and chased the Monster and Niemann into the swampy marshland nearby. They set fire to the marsh grass, forcing the Monster deeper into the bog. The barely conscious Niemann recognized that they were entering a patch of quicksand and tried to warn the Monster, but the creature kept walking deeper into the quicksand until they both sank beneath the surface. Although the Monster would later return to life, Doctor Niemann was dead.
- Niemann's first name, Gustav, was revealed on the front of a sign outside his laboratory in Visaria.
- Niemann's relationship to Frankenstein's assistant Fritz, establishes Fritz's surname as Niemann as well.
- House of Frankenstein is the first Frankenstein film to star Boris Karloff where he does not play the monster.
- The reasons why Doctor Niemann wants to place the mind of the Frankenstein Monster inside Larry Talbot's body are never revealed.