An unknown individual breaks into the Terzi Medical Institute but seemingly takes nothing. One of the Institute doctors, Calabresi, confides to his fiancée, Bianca Merusi, that he knows who broke into the Institute and why. He attempts to blackmail the individual, but the assailant pushes him in front of an arriving train, killing him. A
paparazzo captures Calabresi's fall, but not the killer. Reporter Carlo Giordani has been covering the break-in investigation and writes an article about Calabresi's death, including the photo. Franco "Cookie" Arnò, a middle-aged blind man who was once an ace reporter, and his niece Lori visit Carlo after reading the article. Franco has a hunch that someone cropped the newspaper photo, and a call to the photographer confirms this. However, after they ask the photographer to print the entire picture, someone strangles him to death. The killer takes the photo and all the negatives before Carlo, Franco, and Lori arrive. Carlo, impressed with Franco, lets him assist his investigation. Discussing the case, they observe nine leads: the five remaining Institute scientists (Mombelli, Esson, Casoni, Braun, and director Terzi), Terzi's daughter Anna, Bianca, the original break-in, and the missing photographs. They joke that the case resembles a
cat o' nine tails and resolve to follow each lead. Carlo interviews Anna, who reveals that the Institute has been researching "
XYY syndrome". Their study suggests that people with the XYY chromosome have a "criminal tendency". Meanwhile, Franco and Lori meet with Bianca, who provides no additional information, but Lori tells Franco that Bianca was nervously fiddling with a
locket as she spoke. That night, Bianca searches Calabresi's car and finds a note detailing the killer's identity. She hides the note in her locket. Bianca returns to her apartment, where the killer strangles her. The killer searches her but cannot find the note, which is hidden in the locket. Despite receiving a threatening note from the killer, Carlo and Franco continue investigating. Carlo speaks with other Institute doctors. Dr. Mombelli reveals that everyone at the Institute submitted blood samples to be tested for the XYY research. At the same time, Dr. Casoni theorizes that testing for XYY may become a method for crime prevention. That night, Carlo and Franco both avoid separate attempts to kill them. Carlo tries to look for Dr. Braun, whose escape made him a suspect, but at his boyfriend's house, he is found stabbed to death. Franco tells Carlo that Bianca must have died because the killer suspected she had evidence, and, remembering Lori mentioning Bianca's locket, he deduces that the proof may be in the locket. They discover that Bianca has been buried with the locket, so they go to her family crypt and search her coffin. Inside the locket, they find the folded note, but before they can read it, the killer shuts the crypt door, locking Carlo inside and attacking Franco outside. The killer takes the note, but Franco stabs him with his
sword cane, causing the killer to flee. The killer calls Franco and Carlo, revealing that he has kidnapped Lori and will kill her unless they stop investigating. Knowing that the killer will kill Lori anyway, they call the police. Franco, Carlo, and the police rush to the Terzi Institute to search for Lori, but they cannot find her. Carlo follows a trail of blood to the roof and finds Casoni, the killer, still bleeding from Franco's attack. Casoni prepares to stab a bound and gagged Lori, but Carlo leaps in front of her and gets stabbed in the shoulder. The police arrive on the roof and chase Casoni. Franco stops him with his cane blade; Casoni confesses that he initially broke in to replace the records that showed he tested positive for the XYY chromosome. When Franco asks about Lori, Casoni lies to Franco that he killed her. Enraged, Franco knocks him through a
skylight and down an elevator shaft to his death as a now-free Lori calls out for Franco. ==Cast==