The Girl Who Knew Too Much

American tourist Nora Davis arrives in Rome to visit her elderly, ailing aunt who is receiving treatment from Dr. Marcello Bassi. Nora witnesses her aunt's death on her first night in the city, which sets the stage for the unfolding mystery. She then walks to the hospital to notify Dr. Bassi about her aunt's passing.

On her way to the hospital, Nora is mugged and knocked out in Piazza di Spagna. When she regains consciousness, she witnesses a young woman being murdered outside a house. She also sees a bearded man pulling a knife from the victim's body, which leaves her traumatized and confused about the crime.

Nora witnesses the murder of a young woman and sees the killer pulling a knife from her corpse. She faints from the shock of the scene. The police do not believe her report due to a lack of evidence, leaving her feeling isolated in her account of the incident.

Nora attends her aunt's funeral, where she meets Laura Craven-Torrani, a close friend of her aunt. Laura offers Nora to stay at her house in Piazza di Spagna as she prepares to leave for Switzerland the next day. This introduces Nora to new contacts and potential clues about the murders.

At Laura’s house, Nora finds newspaper clippings about the 'Alphabet Killer,' a serial murderer targeting women in alphabetical order by their last names. The killer had previously murdered Laura’s sister ten years earlier, which personalizes the case for Nora and intensifies the threat.

Nora receives a mysterious phone call mocking her, saying that ‘D’ is for death. This taunt confirms that the killer is targeting victims in alphabetical order and signals that Nora might be the next target, heightening her anxiety and suspicion.

Dr. Marcello Bassi takes Nora on a sightseeing tour of Rome to distract her from the murders. Their relationship develops romantically during the trip, adding a personal layer to the unfolding investigation and emotional stakes.

Nora is lured into an empty apartment where a voice on a tape recorder warns her about the letter 'D.' It is discovered that the apartment is leased to investigative reporter Andrea Landini, who has been secretly monitoring Nora and Marcello, raising suspicions about his involvement.

Nora visits Landini’s apartment and finds clues that suggest he may be the killer, including a confession note, a photo of Nora with a label indicating she might be the next victim, and the tape recorder. Shortly after, she finds Landini dead, apparently having committed suicide, which complicates the mystery further.

Nora learns that the latest victim’s body is identified as Straccianeve’s daughter, confirming the pattern of the alphabet murders. This discovery ties the recent murders to the historical killings and deepens the mystery surrounding the killer’s identity.

Laura returns from her trip and is revealed to be the murderer. She confesses to killing her sister for inheritance and her husband for protecting her. Laura’s descent into madness and her criminal actions come to light during a tense confrontation.

Laura attempts to shoot Nora but is shot dead by her dying husband, who was trying to protect Nora. The revelation of Laura as the killer concludes the investigation, resolving the danger Nora was in, and providing justice for her sister’s murder.

With the case closed, Nora is reunited with Marcello, and they plan to marry. The story ends with a sense of hope and new beginnings, as Nora finds closure and safety after her traumatic experience.