Having recently moved, twenty-seven-year-old Lori Bancroft inquires at a small urban apartment building about an advertised vacancy. The landlord and building superintendent Karl Gunther, an older German man, hospitably gives her a tour of the apartment, telling her that its last tenant was a young woman who disappeared without paying rent. During the tour, Gunther secretly performs a masochistic rite, holding his hand over an open stove-top flame.

Outwardly normal, Gunther leads a double-life as a sadistic, self-loathing psychopath, abducting and torturing his young female tenants and locking them in attic cages, where he removes their tongues and leaves them alive so that he can "have someone to talk to." Once a respectable doctor, he made his living performing euthanasia ("mercy killing") and being ashamed when he learned that his father, a Dachau concentration camp doctor, used the same justification when killing Jewish prisoners in human experiments. Besides regular self-harm, Gunther plays Russian roulette with a loaded handgun, hoping to one day kill himself to end his killing spree with what little morality he has left.

After murdering one of his tenants and removing their eyes, Gunther is visited by Josef Steiner, who has been searching for him for three years. Steiner tells Gunther that in the five years he was chief resident at Buenos Aires hospital, sixty-seven people in his care all died, including Steiner's own brother. He confronts him about his familial history among Nazis, including how his father was executed for crimes against humanity and a photograph of young Karl in a Hitler Youth uniform.

Karl begins spying on and murdering his tenants via the reinforced ventilation crawlspace vents, and a series of mechanized traps he controls from his residence. Like his father, he begins displaying signs of a God complex, reveling in the ability to give life and take it away at will. Steiner attempts to assassinate Gunther, but is instead led into his apartment, where he is killed by one of his traps. Gunther proceeds to pose in an S.S. uniform in front of a mirror and declaring himself his "own god, own jury, and own executioner."

Lori returns home to her apartment to find her refrigerator swarming with live rats and Steiner's corpse in the bathtub, a swastika carved into his forehead. Lori tries to run for help as Gunther sets off security mechanisms that trap her inside the building. Running from door-to-door, she finds her neighbors all killed in similarly brutal fashion. Lori flees into Gunther's attic hideout, where she finds his last surviving caged female prisoner. As Gunther approaches, she manages to sneak through a booby-trapped crawlspace vent. Gunther releases a cage full of rats into the vent after her, but she manages to avoid them and circle back to his room. Gunther pursues her, but appears to inadvertently set off one of his own traps and impale himself with a blade, Lori and the female prisoner taking the opportunity to run away. The gaff, however, is only a ploy. As the two run to Karl's apartment to phone the police, he chases them with a knife. Lori grabs Karl's revolver and fires it at him, it clicks empty several times before finally shooting its only round. Karl accepts his death before expiring, declaring "so be it." (fonte: Imdb)

Karl Gunther è un medico in pensione che possiede vari appartamenti che affitta a ragazze attraenti. Infatti, durante la notte, si diverte a spiare le inquiline dai condotti dell'aria fra le pareti, provocando un misterioso "toc toc" che loro non sanno spiegarsi. Ma Gunther, ben presto, non si accontenterà più solo di spiare... (fonte: retro del dvd)

Un uomo che gestisce un appartamento per le donne è il figlio pazzo di un chirurgo nazista che ha la casa equipaggiata con passaggi segreti, stanze nascoste e strumenti di tortura e omicidio.