In 1956, during a chilling Christmas gathering at a family home, an unseen assailant commits a horrific act, stabbing a victim to death. The scene becomes even more unsettling as a bloody knife clatters to the ground, perilously close to a child's feet.
Fast forward twenty years later in Turin, the diligent Professor Giordani presides over a parapsychology conference featuring the enigmatic psychic medium, Helga Ulmann. In the midst of her presentation, Helga is suddenly overcome with a dark sensation, sensing the "twisted, perverted, murderous" thoughts of an audience member. After the conference, she confides in Giordani about her alarming revelation, completely unaware that a shadowy observer lurks nearby.
That very night, a black-gloved figure stealthily breaks into Helga's home, ruthlessly taking her life with a meat cleaver. Marcus Daly, an English jazz pianist, happens to witness the gruesome incident through the window as he walks by. Rushing inside, he is confronted by a gruesome sight: Helga's mutilated body lying cold. When the police arrive, Marcus is left with an unsettling feeling that something from the apartment’s wall — a painting, perhaps — is amiss, although he can’t quite pinpoint what’s gone.
The following day, the media spotlight falls on Marcus as the known eyewitness, showcasing a photo of him taken by reporter Gianna Brezzi. Seeking solace, Marcus visits his hard-drinking friend Carlo, but finds himself instead face-to-face with Carlo's unconventional mother, Martha, who seems oddly fascinated by him. As night blankets the city, Marcus hears a sinister recording of a child’s song echoing outside his door. He narrowly locks the door shut before a threatening whisper penetrates the silence: “I'll kill you sooner or later.”
Haunted by guilt for potentially endangering Marcus with her report, Gianna decides to assist him in unraveling the mystery. They convey their findings to Giordani, who recalls that Helga spoke of hearing a similar child's song during her eerie vision. He mentions a book on local folklore that describes a haunted house linked to such haunting sounds. Marcus finds this book at the library, and upon tearing out a picture of the notorious haunted house, he plans to meet its author, Amanda Righetti to uncover more. However, unbeknownst to him, the killer is already stalking him and attacks Amanda, subjecting her to a horrific drowning before Marcus lays eyes on her.
Determined, Marcus uses the picture to locate the dilapidated edifice, where he uncovers a chilling mural depicting a child brandishing a bloody knife over a lifeless body. Upon leaving the site, a piece of sheetrock falls, hinting at more than meets the eye. Meanwhile, as Marcus delves deeper into the investigation, Giordani, who had been aiding him, falls victim to the merciless killer, slain after being distracted by an unwieldy mechanized doll.
In the abandoned house, Marcus discovers a concealed room, within which lies a corpse long forgotten. Just as he recoils in shock, he is struck unconscious by an unseen assailant. He regains consciousness outside the house, now engulfed in flames. Gianna explains that she deciphered his urgent message about the investigation in time to rescue him. While waiting at the caretaker's residence for the authorities, Marcus becomes aware of the caretaker's daughter, who has created a drawing eerily similar to the one he found in the haunted house. She reveals that she had seen it in the local school archives.
Eager for answers, Marcus and Gianna make haste to the school. Locating the drawing in a record associated with a schoolboy, their investigation turns dire when Gianna exits to summon the police and is followed by a sudden attack. In a tense standoff, Marcus confronts the assailant, only to discover it is none other than Carlo, whose childhood brush with the trauma compels him to shield his mother. The police arrive just as Carlo escapes into the night, but his fateful end arrives when a garbage truck drags him into the dark.
At the hospital, Marcus learns that Gianna has survived the attack, igniting a spark of hope. Reflecting on the chaotic sequence of events surrounding Helga's murder, Marcus recalls a key detail: the reflection of the real killer framed in a mirror at the scene. Upon realizing it was Martha, Carlo's troubled mother, he understands the tragic truth. Confronting Martha reveals a disturbing backstory; after murdering her husband in a fit of rage before young Carlo, she had hidden his body away, psychologically scarred the boy, leading him to repress those memories through alcohol.
As Martha attacks Marcus with a meat cleaver, their violent struggle culminates in a shocking twist. In a final act of survival, Martha gets entangled in the elevator's bars, leading to her demise as Marcus sends the elevator plummeting, decapitating her in a brutal turn of fate.
