An American writer in Rome is stalked by a serial killer bent on harassing him while killing all people associated with his work on his latest book.

Peter Neal: I've been charged, I've tried building a plot the same way you have. I've tried to figure it out; but, I just have this hunch that something is missing, a tiny piece of the jigsaw. Somebody who should be dead is alive, or somebody who should be alive is already dead.
Detective Germani: Explain that.
Peter Neal: You know, there's a sentence in a Conan Doyle book, "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."
Detective Germani: We have eliminated the impossible. Whatever remains, however improbable, must be the truth.
Peter Neal: Let me ask you something? If someone is killed with a Smith&Wesson revolver... Do you go and interview the president of Smith & Wesson?
[First lines]
Narrator: "The impulse had become irresistible. There was only one answer to the fury that tortured him. And so he committed his first act of murder. He had broken the most deep-rooted taboo, and found not guilt, not anxiety or fear, but freedom. Any humiliation which stood in his way could be swept aside by the simple act of annihilation: Murder."