Dario Argento's debut is a Hitchcockian chiller rather than one of the splatter gorefests with which he was later to make his name. Sam Dalmas, an American writer in Rome, witnesses an attempted murder and finds himself under suspicion. When he's attacked himself, the police realise his innocence, but Dalmas has become intrigued by the assault, which is the work of a serial killer, and sets out on the killer's trail.