Dr. Gustav Niemann is a mad scientist and the main antagonist in the 1944 horror film, House of Frankenstein, which is the sequel to Frankenstein Meets the Wolf Man.
He was portrayed by the late legendary horror actor, Boris Karloff, who is most famous for playing Frankenstein's Monster.
The brother of Dr. Frankenstein's assistant, Niemann was trying to follow in the footsteps of Dr. Frankenstein but was imprisoned for graverobbing and spent 15 years in prison. In prison he befriended a hunchback named Daniel. A lightning storm destroys the prison, allowing the two to escape. They meet a traveling showman named Bruno Lampini who had the remains of Dracula. When he refuses to take Niemann and Daniel to Reigelberg, Niemann has Daniel kill Bruno and Niemann takes Bruno's place. At Reigelberg, Niemann shows off Dracula's skeleton and sees Herr Hussman, one of the men who sent him to prison. After the show ends, Niemann takes the stake out of Dracula's remains in an attempt to kill Hussman but removing the stake revives Dracula. Niemann keeps the stake near Dracula's heart and because Niemann's willpower was strong enough to be immune to Dracula's hypnosis, Dracula makes a deal with Niemann in return for the safety of the coffin. Dracula kills Hussman and kidnaps his granddaughter. The granddaughter's husband calls the police who chase Dracula. Realizing the police were after Dracula, Niemann has the coffin removed from the caravan. Unable to open the coffin, Dracula succumbs to sunlight and dies again.
After reaching the village of Frankenstein, Daniel falls in love with a gypsy girl named Ilonka, who comes with them as Niemann searches the remains of Frankenstein's castle for Frankenstein's notes. They uncover Frankenstein's monster and The Wolf Man. Believing one of the two would know the location of Frankenstein's notes, Niemann thaws the two out, but the monster remains dormant. Larry Talbot, upset that he was thawed because it kept his werewolf form at bay, confronts Niemann. Niemann says he would fix his curse by placing Talbot's brain in a new body if he got Frankenstein's notes. Talbot agrees and shows where the notes are. They then grab Frankenstein's monster and the group travel to Visaria, where Niemann's laboratory was.
During this time, Ilonka falls in love with Talbot, making Daniel jealous. Daniel tells Ilonka that Larry was a werewolf and though she believes Daniel, she yells at Daniel and calls him ugly. At Visaria, Niemann shows more interest in reviving the Monster than helping Daniel or Talbot. Talbot turns back into the Wolf Man and attacks Ilonka. She shoots him with a silver bullet, but she dies in the attack.
Daniel anxiously blames Niemann and said if Niemann kept his promises, Ilonka would have lived. Daniel strangles Niemann to near unconsciousness, but the Monster awakens and throws Daniel out of a window to his death. Already, an angry mob shows up so the Monster takes a semi conscious Niemann out of the laboratory to the woods. The angry mob corner the two near some quicksand. Niemann takes notice and tries to warn the Monster but the Monster walks into it. Unable to escape, both the monster and Niemann sink, ending the film. In the sequel, House of Dracula, the Monster is still holding onto Niemann, who is now nothing but a skeleton.