A young woman walks through the hallways of an apartment building, looking for "Mr. Gunther". She walks into a room in the attic and the door closes and locks behind her. Inside the room, she finds rats in cages, as well as a woman (Sally Brown) in cage. Karl Gunther (Klaus Kinski) appears and says of the caged woman: "She can't talk. I cut her tongue off." Karl presses a button and a blade is shoved through the woman's back and sticks out of her chest, killing her. Karl smears some of the dead woman's blood from his finger into an inscribed bullet and loads it into a revolver. He points the gun to his head and the gun clicks. He puts the gun down and says: "So be it."Sometime later, Sophie Fisher (Tané) is in her apartment in the building, which is located in "a big city" somewhere in the USA. A man watches Sophie from her outside window on the ground floor, and Karl is seen also spying on her from behind an air vent. The man crawls through the window and surprises her, but it is only her boyfriend Hank (David Abbott), whom has come to visit her. Hank and Sophie kiss and spend time on her bed as they begin to have sex while Karl continues to watch them.The next day, a man comes to Karl's door of his apartment where he inquires about a vacant apartment advertised in the local newspaper. Karl takes one look at the man and tells him that it has been rented. But seconds after the man leaves, Lori Bancroft (Talia Balsam) enters the small urban apartment building and when she inquires about the vacant apartment, Karl hospitably takes her to it and shows her around the room. Karl introduces himself as the landlord and superintendent of the building and that the apartment was recently vacated by a young woman who disappeared without paying her rent. Karl explains about the building, the neighborhood and of the monthly rent. As Lori looks around the kitchen, Karl turns on the stove and puts his hand over the blue flame while Lori is not looking. Hiding his pain, Karl asks if she will take the apartment. Lori says yes, which prompts Karl to pull his hand away from the flame and shakes her hand with his other one. Later, Karl is back in his secret room in the attic writing in his diary and says that he is addicted to killing for it makes him feel alive. Martha, the tongueless woman in the cage, gives him a note saying: "Please kill me". Karl tells Martha that he cannot because he would have no one to talk to.The following day, Harriet Watkins (Barbara Whinnery), another resident of the apartment complex, arrives at her apartment with some groceries. Karl appears in the hallway and helps her with her bags. Seeing the bandage on his right hand, Harriet is about to ask him what happened and Karl tells her that he accidentally burned himself. Harriet talks to him about "vices" but he seems reluctant to discuss it.Later, all of the women of the building have a get-together to welcome Lori while Karl spies on them from the air vents. He pushes a few buttons on a remote control device and a small door opens in the room and a rat comes out. The women scream and jump up on furniture, but Lori just sits and laughs at it all.Back in her apartment, Lori hears clicking sounds and goes to Karl's front door. She tells him about the sounds, and he says that its probably just rats. In his diary, Karl writes of once being a doctor, practicing euthanasia ("mercy killing") and being ashamed at himself when learning, after reading his father's secret diary, that the Nazis used the same word when killing Jews. Karl plays Russian roulette with his gun hoping to someday kill himself to end his killing spree with what little morality he has left.A little later, Sophie is in her apartment playing her piano and singing, while Karl watches her from outside her window. He ducks away when he sees Hank. Hank notices Karl nearby and correctly assuming that he is also spying on Sophie, he moves in with a switchblade, while Karl pulls out a switchblade knife of his own. Later, Karl is back in his apartment where he puts two eyeballs into a jar of formaldehyde. He goes to his back room, points the gun at his head and pulls the trigger and it clicks. He tries the gun again and gets another click. "So be it," says Karl.A few days later, Josef Steiner (Kenneth Robert Shippy) arrives at the building and goes to Karl's front door. When Karl answers, Steiner claims that he has been searing for Karl for almost three years. Karl reluctantly lets him inside, and Steiner calls him a murderer. He says that in the five years that Karl was the chief resident at a hospital in Buenos Aires, Argentina, 67 people under his care (all with routine illnesses) died, one of them being Steiner's brother. Steiner shows a photograph of Karl's father to him, dressed in a Nazi SS uniform, and some papers where Karl's father described instruments of torture for concentration camps and was executed over 30 years ago for "crimes against humanity". Karl tells Steiner to leave, and the man brings out a photograph of a young Karl, saluting in a Nazi uniform. When he is alone, Karl weeps.Later, Karl writes more in his diary about accidentally killing a healthy patient when he was a resident at a hospital in Nazi Germany in the early 1940s, and feeling no remorse. He began to understand the feelings that his equally sociopath father wrote about: it was god-like to the able to give life and then take it away.In his crawlspace by the vent, Karl spies on Jessica Marlow (Carl Francis), a local soap opera actress, who arrives back at her apartment with her boyfriend Alfred Lassiter (Jack Hiller). Karl begins to make clicking sounds in the vent by hitting his knife on a steel ball, and continues doing so after Jessica and Alfred are in bed having sex. Karl hears a sudden, sharp scream and he quickly crawls away and back into his apartment only to see that a pet cat of his had inadvertently set off one of his traps, leaving only a detached tail behind. "Sorry, kitty", says Karl.Back in Jessica's apartment, Alfred becomes irritated by the clicking sounds and leaves. He hears something outside the hall and goes into Karl's room. A little later, Karl is seen putting a severed finger (with Alfred's class ring on it) into another jar of formaldehyde.As Lori is preparing to take a bath, she hears clicks, and a large rat jumps out at her from the vent in the bathroom. She goes to Karl's front door and knocks but there is no answer. In his room, Karl crawls back and is setting up another trap, pushing a button on the arm of a chair and a spear shoots up from the seat.Another few days later, Steiner goes to Lori's apartment and tries getting information from her about Karl and how much she knows about him. Karl overhears them from his crawlspace behind the air vent and loads a dart gun and prepares to use it to shoot Steiner, but he changes his mind (surmising that it would not be painful enough). Lori asks the nosy Steiner to leave. He then heads up to Karl's door which is open and lets himself inside. Steiner sees swinging steel balls hanging from the ceiling. He sits in a chair and reads from Karl's diary. Karl appears in the room asking what he is doing. Steiner sees him, turns, and accidentally sets off the chair trap. Looking distraught, Karl runs to his back room, loads a single bullet into the revolver, spins it, and pulls the trigger three times only to get three clicks. "So be it," says Karl. He then goes up to his secret room in the attic and runs a film projector showing a movie of Hitler and the Nazis at a political rally. Karl proclaims: "I am my own god, my own jury, and my own executioner". He puts on an old SS uniform with a Nazi cap, looks at himself in a mirror and salutes: "Hail, Gunther!" He decides that now is the time to kill everyone who knows about him before the police show up looking for Steiner and the other murder victims.Later that same night, Lori returns home to her apartment and finds live rats in her refrigerator. Just then, Karl phones her where he tells her to look in her bathtub. Lori enters her bathroom and finds a dead Steiner floating in the blood-stained water with a swastika carved on his forehead. Lori runs out and sees Karl standing outside her window, wearing his Nazi uniform, with his face decorated up looking pasty white complete with blood red lipstick. He is hitting a small steel ball with a knife. Lori tries to run, but steel bars come down before her in the hallway, preventing her from running out of the building. Lori knocks on Sophie's door, forces it open, and finds Sophie dead, sitting at her piano having set off one of Karl's traps. Lori also finds Harriet dead in her apartment too from another one of Karl's traps.Lori runs upstairs to the secret room in the attic and looks for a place to hide. She finds Martha in her cage who points Lori to a hanging key. Lori tries to unlock the cage, but the two women hear Karl coming towards the front door. Martha points Lori to one of the crawlspace entrances in the floor, but that it is booby trapped. Lori takes off one of her sneakers and puts it in the entrance frame on the floor where a sensor activated blade slices off the top of the sneaker. With the trap sprung, Lori is able to enter the crawlspace and begins searching for a way to escape.Karl enters the room and releases a cage of rats into the crawlspace. Lori crawls around to evade the rats and goes to another crawlspace exit only to find herself in Jessica's apartment with a dead Jessica hanging from the ceiling after being caught in another of Karl's booby traps. Karl gets inside the crawlspace and begins crawling after Lori, and eventually speeds after her with a wheeled grate kept in the crawlspace. Lori manages to make it back to Karl's room again and gets Martha out of her cage with the key. Karl crawls out of the crawlspace and hides. Just then, Lori and Martha hear a reaction from Karl. They find Karl with a blade sticking from his chest before he collapses. Lori and Martha run out of the room. Karl's eyes begin to move, and he removes the "blade" from his chest. (This act was staged by Karl apparently to try to lure Lori or Martha close enough so he could kill them... which did not work.)Lori and Martha enter Karl's apartment where they see a phone. When Lori picks up the phone to call the police, she and Martha notice Karl standing in the doorway with a knife. As he approaches the two women, Lori sees the revolver that Karl plays Russian roulette with and grabs it. She points it at him and the gun clicks several times until finally there is a gunshot."So be it," says Karl.
Output: Question: Who crawls through the window and surprises Sophie?, Answer: Hank