When a female tenant searches for apartment manager, Karl Gunther, in the attic, she sees several cages containing rats. Silently, a deadbolt automatically locks the door behind her. She notices a mute girl named Martha White locked in a cage, gesturing for help. Karl appears, and explains that he cut out the girl’s tongue, and points to it, floating in glass jar suspended in liquid. He presses a button that automatically impales his tenant with a spear. Martha witnesses the murder, and cowers in her cage. Karl sits at a table, loads a bullet engraved with his name into the chamber of a gun, and plays Russian roulette. When he is spared, he murmurs, “So be it.” He then posts an “apartment for rent” sign. Later, Karl spies on tenant Sophie Fisher, dressed in red lingerie in her apartment. An intruder with a knife confronts her, and orders her to continue undressing. The intruder is actually Sophie’s boyfriend, Hank Storm, and she wishes he would use the front door instead of sneaking in her window. They make love, and Karl watches the whole episode. Soon, a college student named Lori Bancroft asks to see the apartment. She thinks it is a great improvement over her last place, where her landlord seemed like a vampire, and agrees to rent the apartment. Later, Karl writes in his journal that he used to kill in the name of science, but now he kills because he is addicted. When he feeds Martha through the bars of her cage, she hands him a piece of paper, requesting in writing that he kill her. He says he cannot comply because he would have no one to talk to. Later, another tenant, Harriet Watkins, asks Karl to help carry her groceries. She flirts and invites him in for a drink. He claims he never touches alcohol, has had unsuccessful relationships and hates women. Still later, Karl spies on a welcome party for Lori, thrown by Harriet and the other women renters in the building. From a hand-held device, Karl releases a rat into Harriet’s apartment and breaks up the festivities. In his journal, Karl says he was chief surgeon at the National Hospital when he began practicing euthanasia. He liked the idea of relieving patients of their pain. In 1971, he read his father’s diaries and discovered, to his horror, that his father was responsible for exterminating Jewish people, but rationalized his actions by calling it “euthanasia.” One evening, Karl kills Hank when he appears outside Sophie’s window. He dismembers the body, and puts the body parts into glass jars in his attic workroom. Karl plays another round of Russian roulette with his revolver, but is again spared. One day, Karl receives a surprise visit from Josef Steiner, who claims to have searched three years to find him. As the men talk, Josef believes Karl is a murderer, responsible for the deaths of sixty-seven patients with routine ailments who were admitted to the National Hospital in Buenos Aires, where Karl was chief resident. One of the patients killed was Josef’s brother. Karl expresses sorrow over the brother’s death, but explains that during twenty-seven years of practicing medicine, it was not always possible to save every patient. When Josef pulls a magazine article out of his briefcase, describing the execution of Karl’s father for Nazi war crimes, Karl insists Josef leave. As he goes, Josef hands Karl a photograph of Karl as a boy dressed in a Hitler youth uniform. The memory brings Karl to tears. Josef warns he will be watching Karl. In his workroom, Karl creates more torture and killing devices. He believes killing is what he was meant to do. It makes him feel like a god to take away life. One evening, tenant Jessica Marlow and her companion, Alfred Lassiter, have drinks in her apartment after a date. Karl hides in the ductwork, and spies on the lovers, tapping his knife. Jessica explains the building has mice. Karl interrupts their lovemaking with more tapping. Alfred tells Jessica he cannot make love with the strange sounds in her apartment. He suggests her landlord hire an exterminator and leaves. In the hallway, he follows the noise through the open attic door. There, Karl kills him, and places his finger in a glass jar. He documents Alfred’s murder in his journal as he has done with every killing since he relocated to America. Later, Josef Steiner talks to Lori. He shares his suspicion that Karl is a murderer, which makes her uncomfortable. When she asks Josef to leave, he warns that she is not safe in the building. Karl overhears their conversation. In the hallway, Josef investigates a tapping sound coming from Karl’s apartment. He walks through the open door, and reads pages from Karl’s open journal on a table. Karl appears from behind a curtain, and activates a steel spike that impales Josef in the chair where he sits. Later, Karl plays Russian roulette several times, but is spared. He applies black eyeliner and smears red lipstick on his lips. Then, he watches a newsreel about Nazi Germany, and declares himself to be his own god, jury, and executioner. He wears a Nazi soldier’s hat, and salutes the screen. Later, Lori returns from class, and discovers rats crawling inside her refrigerator. An anonymous telephone caller asks who is swimming in her bathtub. She enters the bathroom, and finds Josef dead in the bathtub with a swastika carved into his forehead. As she runs away, she sees Karl staring at her through the window. She runs to Sophie for help, but she is also dead and propped up at her piano. Next, she runs to Harriet’s apartment, but Harriet has been gagged, and bolted to the wall with a swastika carved on her cheek. Horrified, Lori runs to the attic, and locks herself inside, thinking she will be safe. As she looks around, Martha cries to be freed. They fumble with the key. Martha conceals it, while Lori hides in the ducts when she hears Karl’s footsteps. When Karl narrowly misses wounding Lori with a spear, she crawls through the ductwork, and Karl sends rats to attack her. As she crawls, she notices there is a view into every apartment. Karl lowers himself into the passageway to find Lori. She turns around, and sees Karl rapidly approaching on a dolly. She races to an opening that allows her to climb into the attic, where Karl’s Nazi newsreel plays. Lori hides among the detritus. Martha finds her in the dark and the two women venture out when it appears Karl is accidentally impaled by one of his torture devices. However, as Lori walks with Martha to Karl’s apartment, Karl, still alive, removes a phony spike from his chest. Covered in blood, Karl appears at the front door of his apartment as Lori telephones police. She drops the phone and grabs Karl’s revolver. He laughs because the gun appears to have no bullets, but finally Lori fires a shot that kills Karl. As he collapses, he murmurs, “So be it.”