Young poetess Rose Elliot buys a book from a local antique dealer, a diary in Latin of an architect, E. Varelli. She learns of the Three Mothers, and believes her apartment building is one of their houses. She pleads her brother Mark, who is studying musicology in Rome, to come, because she is afraid. Mark's friend Sara reads her letter, which he left behind in class, and discovers the school is run by the Mater Lacrimarum, and is murdered for this knowledge. The house of Mater Suspiriorum has already been destroyed, and by the time Mark arrives in New York City, he is investigating his sister's murder.
Three years after the incident of the dance academy in "Suspiria", a new group of people are being watched in a New York City apartment building by a mysterious and evil force hell bent on revenge! When young poetess Rose Elliot finds out too much about the place (via an ancient book called "The Three Mothers"), she gets beheaded! The only person to stop the killings and uncover the secrets is her brother Mark, a music student in Rome. As he solves the puzzle, the murders get intense, and the demonic evil starts to reveal itself. As the unstoppable horror spreads from Rome to New York City, this unholy trinity must be stopped before the world is submerged in the blood of the innocent.
An American college student in Rome and his sister in New York investigate a series of killings in both locations where their resident addresses are the domain of two covens of witches.
- Rose Elliot (Irene Miracle), a poet living alone in New York City, discovers an ancient book called The Three Mothers. It tells of the existence of three evil sisters who rule the world with sorrow, tears, and darkness. The book, written by an architect named Varelli, reveals that the three dwell inside separate homes that had been specially designed and built for them by the architect in Rome, Freiburg, and New York. Rose suspects that she is living in one of the buildings and writes to her brother Mark (Leigh McCloskey), a music student in Rome, urging him to visit her. Using clues provided in Varelli's book as a guide, Rose searches the cellar of her building and discovers a hole in the floor which leads to a water-filled ballroom. After accidentally dropping her keys into the water, she enters the flooded room. Swimming under the surface, she sees a portrait bearing the words "Mater Tenebrarum" and is able to reclaim the keys. A putrid corpse suddenly rises from the depths, frightening her. She escapes, although a shadowy figure watches her leave the basement.
In Rome, Mark attempts to read Rose's letter during class. He is distracted by the intense gaze of a beautiful student (Ania Pieroni). When the class ends she leaves suddenly; Mark follows, leaving the letter behind. His friend Sara (Eleonora Giorgi) picks up the letter, and eventually reads it. Horrified by the letter's contents, she takes a taxi to a library and locates a copy of The Three Mothers. While looking for an exit, Sara is attacked by a monstrous figure who recognizes the book. She throws the book to the ground and escapes. Later that night, she seeks the company of a neighbor named Carlo (Gabriele Lavia) and both 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 it is the music student, staring at him intently once again.
Mark telephones Rose but is unable to hear her clearly. He promises to visit just before the connection fails. Cut off, Rose sees two shadowy figures preparing to enter her apartment. She leaves through a back door, but is followed. In a decrepit room, she is grabbed from behind by a clawed assailant and brutally murdered.
Upon arriving in New York, Mark meets some of the residents of Rose's building, including a nurse (Veronica Lazar) who is caring for the elderly Professor Arnold (Feodor Chaliapin, Jr.), a wheelchair-bound mute. Mark learns from the sickly Countess Elise (Daria Nicolodi) that Rose has disappeared. Elise explains how Rose had been acting strangely. After the two find blood on the carpet outside Rose's room, Mark follows the stains. He suddenly becomes ill and falls unconscious. Elise sees a black-robed figure dragging Mark away, but the figure suddenly stops and gives chase to Elise. She is attacked by dozens of cats, who bite and claw at her flesh. The hooded figure then stabs her to death. Mark staggers to the house's foyer where the nurse and caretaker (Alida Valli) put him to bed.
The next day, Mark asks Kazanian (Sacha Pitoëff), the antique dealer who sold Rose the Three Mothers book, about Rose. However, the man provides no information. That night, Kazanian drowns several cats in a Central Park pond and accidentally falls into the water. Hundreds of rats from a nearby drain crawl all over him, gnawing his flesh. A hot dog vendor hears Kazanian's cries and rushes over. The man kills Kazanian with a knife.
Carol, the caretaker, discovers the horribly mutilated corpse of Elise's butler (Leopoldo Mastelloni) in the Countess' apartment. Shocked, she 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.
Meanwhile, Mark uses a clue from Rose's letter to discover that beneath each floor is a secret crawl space. He follows the hidden passages to a suite of rooms where he finds Professor Arnold. The old man 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 becomes caught in his vocal apparatus, choking him. Mark frees him, only to be told by the dying man, "Even now you are being watched." Mark follows a shadowy figure watching him from the doorway to a lavishly furnished chamber, where he finds Varelli's nurse. Laughing maniacally, she explains to him with growing intensity that she is Mater Tenebrarum. She suddenly transforms into Death Personified. However, the fire that has consumed much of the building enables Mark's escape from the witch's den. As the structural integrity of Tenebrarum's home fails, debris crashes down on the fiend, destroying her.