If you are not familiar with the term Giallo or have heard of the name, Dario Argento it is important to know two things. Giallo is a 20th century Italian horror genre in film that is notable for being a mix of caper thriller and psychological horror and Dario Argento is often considered one of the masters of the genre. Deep Red is one of his more notable works and is considered one of the cornerstones of the movement.
The plot begins with a psychic getting a vision that someone in the audience is a murderer. This is a good introduction to the ultra-stylized work of Argento, who uses interesting close-up point of views and enjoys threading information throughout the narrative for a big reveal. The scene then jumps to pianist Marcus (played by David Hemmings) and his friend Carlo (played by Gabriele Lavia) walking home and then witnessing a horrific murder take place in a nearby apartment. The murder is none other than of the psychic herself.
Marcus then finds himself involved in the police investigation, both because he is a witness (and therefore had to have overlooked something) and because he also seems to be infatuated with a local reporter, Gianna (played by Daria Nicolodi). The plot, from thereafter is pretty thin. It's done in typical cat and mouse, crime caper style. With each scene you think you have an idea of who the murderer could be, only to have a new suspect introduced.
At the end, Marcus is pitted against the killer until finally their true identity is revealed and it's the person you least expect! It's formulaic, but don't let that detour you!
Deep Red is brilliant in its simplicity. As mentioned, Argento makes especially stylized work to the point where you know it's one of his within the first ten minutes. What Argento does especially well is hone in on visual cues, made notable by camera techniques and uses musical score to his advantage. He uses camera close-up when he wants to tell the audience something directly but does so in a way that triggers the subconscious. For example, often the killer's eyes are zoomed in on which reveals the culprit wears thick eyeliner, instantly making at least three people suspect. He also closes in on black leather gloves (which Argento himself modeled) to highlight both the brutality and thought process of the killer. Finally, the gore, while not as graphic as other works, still expertly holds up in annals of horror. For example, there's a scene in which someone gets their teeth broken on the fireplace that makes American History X look like a cakewalk. Another seen has someone burning in a bathtub. It's all very visceral. Thus, Argento does well to mix the campy with the horrifying.
Aside from the camera, as mentioned Argento also relies heavily on the film's score, which was done the Italian rock band Goblin. The children singing over a strange melody is especially haunting and seems to be a favored mechanism for Argento. It's certainly effective and helps draw the audience to this world within a world. Dario Argento is a master of the genre and no matter what schisms of horror you like best this is definitely one to check out.