A doctor living in Visaria, Dr. Gustav Niemann had studied under Ludwig Frankenstein. He was apparently quite mad, and used Ludwig's teachings regarding brain transplant in his efforts to put the brain of a man into the skull of a dog. He was arrested for crimes against humanity, and incarcerated at Neustadt Prison, far away from Visaria near the town of Reigelburg. After an unspecified amount of time, Niemann and his cellmate Daniel escaped and took over the travelling sideshow known as Professor Lampini's Chamber of Horrors.
Niemann, burning with the desire for revenge against those who had sent him to prison, posed as Lampini and went to the town of Reigelberg. It was there that one of three men he wanted to kill lived: Burgomaster Hussman, formerly the mayor of Visaria. As it turned out, the late Professor Lampini had amongst his show's exhibits the skeleton of Dracula, a stake in his chest. Niemann removed the stake, reviving the vampire and commanding him to kill Hussman, in exchange for watching over his coffin for him.
When the deed was done, Niemann betrayed Dracula in order to escape from the authorities, and prevented him from getting back to his coffin before sunrise. Then it was on to Visaria, where Niemann's two remaining enemies lived: Strauss and Ullman. In the time since his imprisonment, Ludwig Frankenstein had died and the Frankenstein Monster and the Wolf Man had become entombed in ice in the bowels of Ludwig's mansion. Niemann wanted to get his hands on Henry Frankenstein's notes, and so he and Daniel thawed the Wolf Man, and once he had turned back into Larry Talbot, he helped Niemann find the notes and Niemann promised to cure his lycanthropy in return.
Niemann and Daniel also thawed the Frankenstein Monster, as Niemann wanted to removed the damaged brain of Ygor and replaced with that of Ullmann. But no sooner had he set up shop in his old laboratory and begun getting things ready for the transplant procedure, removing the brains of Ullmann and Strauss, then all his false promises to his various companions began to come back to haunt him. Because he kept delaying in helping Talbot, poor Larry turned into the Wolf Man and killed Ilonka, the gypsy girl who had fallen in love with him (but not before she shot him with a silver bullet, hoping to end his suffering).
Daniel, who had also fallen in love with Ilonka, was enraged and blamed Niemann. He attacked his former cellmate and strangled him into unconsciousness. Mere seconds later, the Monster broke free from his bonds and killed Daniel, then grabbed Niemann and drug him from the lab as the building was stormed by angry villagers. They chased Niemann and the Monster into the swamps that surrounded the manor, and the panicked Monster stumbled into some quicksand and sank under. Niemann, the Monster still holding him, was pulled under as well and drowned.