On vacation in Rome, American traveler Nora Davis arrives to visit her elderly ailing aunt, who is being cared for by [Dr. Marcello Bassi]. The aunt dies on Nora’s first night, and Nora hurries to the nearby hospital to inform the doctor. On the way, she is mugged in Piazza di Spagna and wakes to witness a young woman being murdered in front of a house, with a bearded man pulling a knife from the corpse. Nora faints again. Waking in the hospital the next morning, she reports the incident to the police, who are doubtful because there is no evidence of a murder.
At her aunt’s funeral, Nora meets a close friend of her aunt, [Laura Craven-Torrani], who lives in Piazza di Spagna. Laura is planning to travel to Switzerland the next day and offers Nora a place to stay at her house. In Laura’s home, Nora discovers newspaper clippings about the "Alphabet Killer," a serial killer who has targeted victims in alphabetical order by surname. The killer has already claimed three women with last names starting with "A," "B," and "C." The most recent victim was Laura’s sister, killed ten years earlier in front of Laura’s house. Nora receives a phone call from an anonymous taunter, who whispers that >'D' is for death.
With the help of [Dr. Marcello Bassi], Nora begins to see Rome in a new light, and the two grow closer as they share the fear and the urge to uncover the truth. That night, after Marcello drops Nora off at Laura’s home, an anonymous caller lures her into an empty apartment. Marcello follows, and they hear a voice emanating from a tape recorder that cautions Nora to beware since her last name begins with the letter "D." The apartment is leased to investigative reporter [Andrea Landini], who has been secretly shadowing Nora and Marcello. Landini explains that he collaborated with the police while reporting on the Alphabet Murders, helping to arrest a mentally ill man named Straccianeve, whose guilt he doubted. Landini’s pursuit of the truth eventually cost him his job, but Nora decides to help him, hopeful that together they can crack the case. Still, as they search Rome, no solid clues emerge.
Nora visits Landini’s apartment the next day and uncovers clues that tempt her to believe he might be the killer: the tape recorder, a confession note on his typewriter, and a photo of Nora labeled as a potential fourth victim. Soon afterward, she discovers Landini dead, an apparent suicide.
Laura returns to Rome from her trip. Nora reads in the newspaper that the young woman’s body has been found. At the morgue, Nora identifies the body as Straccianeve’s daughter. Alone in the Craven house that night, Nora notices the study door ajar and sees a man rising from his chair—the man she had seen standing over the body earlier. He collapses to the floor, a knife in his back. The deranged Laura confesses to the killings, revealing that she stabbed her husband because she wanted to have him committed to a psychiatric hospital, and that he was merely disposing of the bodies for her that night Nora saw him. She also admits murdering her sister to claim their father’s inheritance. Laura aims to shoot Nora, but is fatally shot by her dying husband. In the end, Nora is reunited with [Dr. Marcello Bassi], and the couple looks forward to marrying soon.
