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Franco ArnÃ² (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. Meanwhile, Dr. Calabresi (Carlo Alighiero) looks at his files in his office and phones someone and agrees to meet with him. Calabresi tells his fiancee Bianca Merusi (Rada Rassimov) that he knows 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, while 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 Giordani wrote the article. The two of them go to see the reporter at the newspaper office and ask if the picture has been cropped. Carlo calls Righetto (Vittorio Congia), the paparazzi photographer who snapped the picture. Righetto goes back to the original and sees a moving hand-arm in the far left of 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 officer, Spimi (Pier 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. Meanwhile, 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 a 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 found out that Casoni was fired from his last job, and Braun received a lot of money. Carlo goes to see Casoni and the doctor talks about the institute's "wonder drug" and the "XYY pattern". Carlo then asks Dr. Mombell about XYY, and the doctor says that everyone in the institute was tested, but their results are confidential.
The killer approaches Carlo's front door and injects two milk cartons, dropped off by the local milkman, with a syringe. Carlo arrives home and brings the milk cartons inside. Anna arrives shortly thereafter and they talk about more about the research and of her results of the XYY test. They end up having sex. Afterwards, Carlo pours a glass of milk from one of the cartons when Franco phones saying that someone tampered with the gas line on his stove, flooding his apartment with methane gas and also may try to kill Carlo. Carlo notices the milk that had bled through the hypodermic needle holes and knocks the glass away from Anna before she can drink it.
The following day, Carlo meets with one of his old friends and informants, Gigi the Loser (Ugo Fangareggi), for help in investigating the Terzi break-in which may have been an inside job. Carlo and Gigi break into Terzi's house and discover that Anna is adopted and (via a diary) that Terzi "adored" the woman. Carlo goes to the police station and learns from Spimi that Bianca often met with Braun and that the cops cannot find the doctor. Carlo runs a story in the newspaper about Braun being a suspect in the break-in, and a former gay lover of Manuel (Braun's new lover) approaches Carlo and tells him where Braun is hiding. Carlo goes over to the apartment where he is attacked by Manuel. Carlo wins the fight, and sees Braun lying dead on the couch.
A few days later, Franco contacts Carlo about Bianca's locket and suggests that the note that she found might still be there. Franco and Carlo head to the cemetery and open Bianca's family crypt. Carlo gets her coffin open while Franco waits by the door. Carlo finds the locket and discovers the note behind a metal plate and hands it to Franco. As Carlo closes the coffin, the killer shuts the crypt door, locking him in, and attacks Franco. The killer takes the note, but Franco stabs him with his walking cane. While Franco reopens the door to let Carlo out, Lori is hit on the head and put in the back of a car. Franco and Carlo find the taxi which the killer and Lori rode and discover blood in the back seat. The killer calls Franco and tells him to stop investigating the break in and murders.
Carlo goes to the police to report the kidnapping and they go to the Terzi house. Anna comes downstairs with cloth wrapped around her hand. Carlo tells her he know about the father-daughter "relationship" and expresses suspicion about the milk incident (Anna had the glass of poisoned milk for some time without drinking it). But Anna claims that she only cut her hand on a broken vase and was nowhere near the cemetery.
Carlo and the police arrive at the Terzi Institute and search the place for Lori, but they find nothing. Carlo sees blood dripping from the ceiling in one room. He climbs up to the roof and finds Casoni, who hits him in the face and kicks him to the ground. Casoni, with a stab wound to his stomach, goes to a back room where a bound and gagged Lori is and prepares to stab her. Carlo runs in and tackles Casoni, but is stabbed in the chest. The police arrive on the roof and chase Casoni. Franco stops him with his cane blade. Franco asks about Lori, and Casoni tells Franco that he killed her. Enraged, Franco swings his cane at Casoni, knocking him through a sky window and down an elevator shaft to his death, just as Lori calls out for Franco.
The Terzi Institute is what type of complex?
Medical