Directed by Dario Argento, Deep Red (1975) stars David Hemmings and Daria Nicolodi in one of the most influential Giallo films ever made. The story follows a musician who witnesses a brutal murder and becomes obsessed with uncovering the killer's identity. Combining mystery, psychological tension and graphic violence, the film helped define Argento's style and remains a landmark of Italian horror cinema.

Arrow Films

YOU WILL NEVER FORGET IT!!! From Dario Argento, maestro of the macabre and the man behind some of the greatest excursions in Italian horror (Suspiria, The Bird with the Crystal Plumage), comes Deep Red - the ultimate giallo movie. One night, musician Marcus Daly (David Hemmings, Blow-Up), looking up from the street below, witnesses the brutal axe murder of a woman in her apartment. Racing to the scene, Marcus just manages to miss the perpetrator… or does he? As he takes on the role of amateur sleuth, Marcus finds himself ensnared in a bizarre web of murder and mystery where nothing is what it seems… Aided by a throbbing score from regular Argento-collaborators Goblin, Deep Red (aka Profondo Rosso and The Hatchet Murders) is a hallucinatory fever dream of a giallo punctuated by some of the most astonishing set-pieces the sub-genre has to offer.

Från IMDB:
A jazz pianist and a wisecracking journalist are pulled into a complex web of mystery after the former witnesses the brutal murder of a psychic.