Plot
Rose Elliot, a poet living alone in NYC, finds an ancient book titled The Three Mothers. The book, written by an alchemist named Varelli, tells of three evil sisters who rule the world with sorrow, tears, and darkness and dwell in separate homes built for them by the alchemist. Mater Suspiriorum, the Mother of Sighs, lives in Freiburg. Mater Lachrymarum, the Mother of Tears, lives in Rome, and Mater Tenebrarum, the Mother of Darkness, lives in NYC.
Rose suspects that she is living in one of the buildings and writes to her brother Mark, a music student in Rome, urging him to visit her. Using clues provided in the book as a guide, Rose searches the cellar of her building and discovers a hole in the floor that leads to a water-filled ballroom. She accidentally drops her keys and enters the water to find them. After she reclaims the keys, a putrid corpse rises from the depths, frightening her into screaming and beginning to drown. Rose gets out of the water and manages to escape, but is still being watched.
In Rome, Mark attempts to read Rose's letter during class. He is distracted by the intense gaze of a beautiful student who leaves suddenly; Mark follows, leaving the letter behind. His friend Sara picks up the letter and later reads it. Horrified by the letter's contents, she takes a taxi to a library and finds a copy of The Three Mothers and takes it with her. Sara is attacked by a monstrous figure in a laboratory within the building, who recognizes the book. She throws the book to the ground and escapes. Returning home, she phones Mark to tell him to come and asks a neighbour, Carlo, to keep her company. The lights go out, and both Sara and Carlo are stabbed to death by a gloved killer. Mark discovers the bodies and two torn fragments from Rose's letter. After the police arrive, he walks out of Sara's apartment and sees a taxi slowly driving by. In the back seat of the vehicle is the music student, staring at him intently again.
Mark telephones Rose but cannot hear her clearly. He promises to visit just before the connection fails. Rose sees two shadowy figures preparing to enter her apartment. She leaves through a back door, but is followed. She is grabbed from behind by a clawed assailant and guillotined across the neck with glass from a broken window.
Arriving in New York, Mark goes straight to Rose's building, where he meets Carol, the concierge, and some of the residents, including a nurse who cares for elderly Professor Arnold, a wheelchair-user who cannot speak. Mark encounters the sickly Countess Elise De Longvalle Adler, who tells him that Rose has disappeared. After the two find blood on the carpet outside Rose's room, Mark follows the trail of blood. He becomes ill and falls unconscious. Elise sees a black-robed figure dragging Mark away, but the figure stops and comes after Elise. While trying to escape, she is attacked by a swarm of vicious cats, and the figure then catches up with her and stabs her to death. Mark staggers to the house's foyer, where Carol and the nurse put him to bed.
Mark asks Kazanian, the antique dealer who sold Rose "The Three Mothers," about his sister's whereabouts. However, the man provides no information. That night, Kazanian drowns several cats in a Central Park pond and falls into the water. Hundreds of rats from a nearby drain crawl over him, gnawing his flesh. A hot dog vendor hears Kazanian's cries and rushes over, but proceeds to kill him with a knife. More strange deaths occur in the building, while Carol and Elise's butler, John, plots to take advantage of the Countess' death by stealing her valuables. A shocked Carol finds John's corpse in Elise's apartment and drops a lit candle, which starts a fire. Attempting to put out the flames, she becomes entangled in burning draperies and falls from a window to her death.
Mark uses a clue from Rose's letter to discover that beneath each floor is a secret crawl space. He follows hidden passages to a suite of rooms where he finds Professor Arnold, who reveals, via a mechanical voice generator, that he is in fact Varelli. He tries to kill Mark with a hypodermic injection. During the struggle, Varelli's neck is caught in his vocal apparatus, choking him. Mark frees him, only to be told that he is still being watched as Varelli dies. Mark follows a shadowy figure to a lavishly furnished chamber, where he finds Varelli's nurse. Laughing maniacally, she reveals to him that she is Mater Tenebrarum. She transforms into Death personified. However, the fire that has consumed much of the building enables Mark to escape from the witch's den. Debris crashes down on Tenebrarum, destroying her.
