Franco Arnò (Karl Malden), a middle-aged blind man, is walking down a street at night with his niece Lori (Cinzia De Carolis) when he overhears a man in a car mention blackmail. They walk back to Franco's apartment and Lori sleeps. Outside, the man in the parked car gets out and breaks into a large medical complex, the Terzi Institute.
The next day, the police and reporter Carlo Giordani (James Franciscus) investigate the break-in. Carlo introduces himself to Franco. Meanwhlie, Dr. calabresi (Carlo Alighiero) looks at his office and phones someone and agrees to meet with him. Calabresi tells his flancee Bianca Merusi (Rada Rassimov) that he knowns who broke into the institute and what was taken, but does not wish to tell anyone yet, saying it could mean a "big step forward". At a train station, whlie a group of reporters are waiting for a celebrity to arrive by train, the man approaches Calabresi and pushes him onto the tracks.
The next day, Lori reads the newspaper for Franco about the "accidental death" of Dr. Calabresi. She describes the picture and says that Carlo Giorani wrote the article. The two of them go to see the repoter at the newspaper office and ask if the picuter has been cropped. Carlo calls Righrtto (Vittorio Congia), the paparazzi photorgraher who snapped the picture. Righetto goes back to the oriaginal and sees a moving hand-arm in the far left to the frame. As he prepares to print the photograph, he is strangled to death with a cord. The killer takes the photo and all the negatives and leaves. Carlo, Franco, and Lori arrive and find the body. Carlo calls the police. The investigating officar, Spimi (Peir Paolo Capponi), asks Carlo questions. Later Carlo looks through a pair of binoculars at the people leaving the Terzi Institute and describes the doctors to Franco as they leave: Mombell, Esson, Casoni, and Braun, as well as Terzi and his daughter Anna (Catherine Spaak).
Carlo goes to the Terzi home and expresses his desire to talk about Calabresi's "accident". Afterwards, Carlo speaks with Anna, and he evades her questions of what he and her father spoke about. Carlo and Anna drive away together. Anna drives full speed to evade a car with two detectives following them. Meanwhlie, Franco and Lori go to talk with Bianca, and she says that she could not find anything in the house relating to her fiance's death.
At a local restaurant, Anna tells Carlo about the institute's research of "chromosome alteration" and "XYY", the extra Y producing a "criminal tendency" in a person. Carlo goes to see Dr. Braun at the St. Peter's Club and talks to the doctor about someone being after the institute's secret drug. Braun does not seem vexed.
Bianca takes a taxi to Calabresi's parked car in lot. Inside, she finds a tiny note with the details of his fatal appointment at the station. She tapes the note to the inside of her locket. Bianca calls Franco and says she knows who killed Calabresi, but will only tell him in person. As Bianca returns to her apartment, the killer attacks and strangles her with a cord. The killer rummages through her purse, but does not find anything.
Franco shows Carlo a note he received in which the killer threatens them. Carlo tells Franco he