An unknown person infiltrates the Terzi Medical Institute, seemingly taking nothing during the break-in. Among the staff is Dr. Calabresi, who confides in his fiancée, Bianca Merusi, about his knowledge of the intruder's identity and intent. In a shocking turn of events, he tries to blackmail the burglar, but the thief tragically pushes him into the path of an oncoming train, leading to his death. A paparazzi photographer captures the moment, but fails to record the identity of the killer.
In the aftermath, reporter Carlo Giordani covers the incident, documenting both the break-in and Calabresi's demise in a published article that features the damning photo. Franco "Cookie" Arnò, a former star reporter now blind, and his niece Lori visit Carlo after reading his article. Franco suspects foul play involving the photographed image, prompting a call to the photographer. Confirming Franco's fears, they learn that the photo has been altered. Tragically, before they can examine the entire image, the photographer is murdered, and the killer makes off with the evidence.
Impressed by Franco's instincts, Carlo invites him to join the investigation. They outline a strategy to tackle nine leads connected to the case: the five remaining scientists at the Institute (Mombelli, Esson, Casoni, Braun, and Dr. Terzi), Terzi's daughter Anna, Bianca, the initial break-in, and the missing photographs. They venture into the case, joking that it resembles a complicated ‘cat o’ nine tails.’
During an interview, Anna discloses that the institute is investigating "XYY syndrome," suggesting a possible link between the XYY chromosome and criminal behavior. Meanwhile, when Franco and Lori meet with Bianca, the interaction reveals little, but Lori notes Bianca’s nervousness as she fiddles with a locket.
That evening, Bianca investigates Calabresi's car, discovering a note that could identify the culprit. She hides this vital piece of evidence within her locket. Unfortunately, upon returning home, she becomes another victim as the killer strangles her, ransacking her belongings in search of the note, which remains concealed.
Despite threatening communications from the murderer, Carlo and Franco persist with their investigation. Carlo’s conversations with other doctors lead him to Dr. Mombelli, who recalls the compulsory blood tests for the XYY research. Dr. Casoni, however, ominously suggests that understanding XYY markers could lead to preemptive crime measures. As the tension escalates, both Carlo and Franco narrowly escape attempts on their lives. In a harrowing twist, Carlo discovers Dr. Braun murdered, further implicating him as a suspect.
Franco deduces that Bianca's death stems from the killer’s belief that she possessed incriminating evidence. Remembering Lori’s previous comment about the locket, Franco speculates that the note may still be within it. Unfortunately, while searching Bianca's crypt, they find that she was buried with the locket. In a frantic effort, they dig into her coffin, but just as they uncover the note, the murderer locks Carlo inside the crypt and confronts Franco outside. A confrontation ensues, and Franco nearly loses his life before managing to wound the assailant.
The killer, now threatening both Franco and Carlo with the life of Lori, reveals he has kidnapped her, forcing them into a precarious position. Realizing the gravity of the situation, they alert the police about Lori's peril.
In a race against time, Franco, Carlo, and law enforcement rush to the Terzi Institute in search of Lori. Despite their efforts, she remains elusive. Following a trail of blood, Carlo finds Casoni, the assailant, who is bleeding from Franco's earlier attack. As the killer prepares to harm Lori, Carlo intervenes, sustaining an injury but allowing the police to arrive just in time.
Franco corners Casoni, who confesses that his initial break-in was motivated by the desire to destroy his testing records for the XYY chromosome. In a fit of rage, Franco avenges the harm done to Lori, sending Casoni to his doom as the now-rescued Lori calls out for him.
