Deep Red is Dario Argento's giallo masterpiece. It is my favorite of all his giallo films. (Giallo (plural gialli) is an Italian 20th century genre of literature and film, which in Italian indicates crime fiction and mystery.)
The film is basically about an English musician, played intensely by David Hemmings, who tries to solve a series of murders after witnessing a powerful psychic medium die at the hands of an unknown killer. The killer is hellbent on keeping the dark secret buried. But, Marcus Daly (Hemmings) is also dedicated in getting to the bottom of it all.
Deep Red is beautiful art on film and it is plain to see Argento's stroke of the brush being fine-tuned in this film in preparation for his later masterpieces.