Chinatown

Released in 1974, Chinatown starred Jack Nicholson as J.J. Gittes, Faye Dunaway as Evelyn Mulwray, and John Huston as Noah Cross.
Though this film was made a good thirty years after the established film noir era, it is boldly reminiscent of that era's tradition.
The movie opens to a scene describing Gittes' profession as a private investigator. Then a woman claiming to be Evelyn Mulwray hires him to follow her husband, whom she believes to be having an affair. Gittes takes the job and makes the discovery, subsequently ruining Mulwray's reputation. When the real Evelyn Mulwray enters the scene to sue Gittes for libel, the man becomes thoroughly confused. He becomes determined to get to the bottom of this mystery - which deepens when Mulwray turns up dead, apparently a suicide.
Gittes' adventure is classic noir and classically entertaining. The cameo by director Roman Polanski is especially enjoyable.