An American writer residing in Rome witnesses an attack on a woman in an art gallery late at night. As the only witness to what is believed to be the latest attack by a serial killer, he finds himself trapped between two glass doors, unable to aid the victim or apprehend the assailant. This event marks the beginning of his obsession with the case. Plagued by the feeling that he may have seen something crucial to solving the crimes, he teams up with his girlfriend to conduct his own investigation, despite the police's warnings to stay out of it.

With the support of his girlfriend, the writer delves deeper into the mystery, uncovering a web of dark secrets and bizarre coincidences. He starts to receive threatening calls and his own life becomes endangered as he gets closer to uncovering the identity of the murderer. Each clue seems to bring more questions than answers, and as the couple's amateur detective work continues, they realize that they are dealing with a criminal far more cunning and dangerous than they initially thought. The line between the hunter and the hunted begins to blur, and the writer must use all his wits to stay alive and solve the puzzle before becoming the next victim.