Rose Elliot, portrayed by Irene Miracle, is a solitary poet residing in New York City who stumbles upon an ancient tome titled The Three Mothers. This mysterious book unveils the existence of three malevolent sisters, each reigning over the world with despair, sorrow, and darkness. Penned by the architect Varelli, the book describes the unique residences crafted specifically for each sister, located in Rome, Freiburg, and New York. Convinced that she inhabits one of these cursed buildings, Rose reaches out to her brother Mark, a music student played by Leigh McCloskey, urging him to come visit her.
As she digs deeper into Varelli's ominous text, Rose explores the cellar of her apartment, where she uncovers a concealed hole leading to an eerie, waterlogged ballroom. In a harrowing turn of events, her keys slip into the murky depths, and as she bravely swims to retrieve them, she encounters a chilling portrait inscribed with "Mater Tenebrarum." Suddenly, a decaying corpse emerges from the water, sending her into a frantic escape, while a shadowy figure lurks just beyond.
Simultaneously, in Rome, Mark grapples with deciphering his sister's letter during class, only to be distracted by the striking gaze of a mysterious student (Ania Pieroni). The moment she unexpectedly leaves, Mark impulsively follows her, leaving the letter behind. His classmate Sara, portrayed by Eleonora Giorgi, discovers and reads the urgent letter, plunging her into terror. She rushes to a library to track down a copy of The Three Mothers, but her search takes a nightmarish turn when she is attacked by a ghastly creature recognizing the book. In a heart-pounding escape, she discards the book only to be met with a deadly fate alongside her neighbor Carlo, played by Gabriele Lavia.
Meanwhile, Mark arrives in New York, where he encounters the residents of Rose's building, including a devoted nurse (Veronica Lazar) tending to the frail Professor Arnold, portrayed by Feodor Chaliapin Jr.. From the eccentric Countess Elise, played by Daria Nicolodi, Mark learns of his sister's peculiar behavior prior to her disappearance. Their search quickly spirals into chaos as they find ominous signs of struggle, including bloodstains leading to a horrifying discovery. Mark's investigation into Rose's whereabouts leads him to dizzying confrontations with darkness and death.
The quest intensifies when Mark seeks answers from Kazanian, a character embodied by Sacha Pitoëff, the antique dealer who once sold Rose the ominous book. Unbeknownst to Mark, Kazanian's fate intertwines with the horror as he meets a gruesome end involving a chaotic encounter with rats. In the subsequent unraveling of mysteries, the caretaker Carol, played by Alida Valli, stumbles upon horrific scenes, including the gruesome remains of the Countess's butler, which lead to tragic events.
Eventually, Mark uses a crucial clue from Rose's letter to unveil hidden passageways beneath the floors of her building, leading him to a haunting confrontation with Professor Arnold, the man behind the mask of Varelli. In a sinister twist, Varelli reveals himself as a harbinger of doom, attempting to silence Mark with a lethal injection. However, as chaos erupts with the arrival of a vengeful Mater Tenebrarum, depicted as a nightmarish entity, Mark narrowly escapes the inferno consuming the witch's lair, setting the stage for a chilling climax.
