It's 1937 Los Angeles. Specializing in cases of cheating spouses, private investigator Jake 'J.J.' Gittes is tailing Hollis Mulwray, whose wife suspects him of marital infidelity. Mulwray is the high profile chief engineer for the Los Angeles Department of Water and Power. He being high profile is because of the chronic drought in the Los Angeles area and the importance of fresh water to the growing community. Most of what Gittes witnesses in following Mulwray are some usual and some not so usual business dealings, among the former being a public meeting for construction of a new dam to create an additional water supply for Los Angeles, the dam which Mulwray opposes. But Gittes eventually witnesses Mulwray meeting with a young unknown woman who is not his wife. Once news of the supposed tryst between Mulwray and this woman hits the media, additional information comes to light that makes Gittes believe that Mulwray is being framed for something and that he himself is being set-up. Gittes is assisted in his investigation of the issue behind Mulwray's framing and his own set-up by Mulwray's wife, Evelyn Mulwray. Despite Evelyn's help, Gittes also believes that she isn't being totally forthright with him. The further he gets into the investigation, the more secrets he uncovers about the professional and personal dealings of the Mulwrays, which includes Mulwray's former business partnership with Evelyn's father, Noah Cross. The identity of the unknown woman may be the key to uncovering the whole story.