A woman identifying herself as Evelyn Mulwray hires private investigator J.J. "Jake" Gittes to follow her husband, Hollis Mulwray, chief engineer for the Los Angeles Department of Water and Power. Gittes tails him, hears him publicly refuse to create a new reservoir that would be unsafe, and shoots photographs of him with a young woman, which are published on the front page of the following day's paper. Back at his office, Gittes is confronted by a woman who informs him she is the real Evelyn Mulwray and that he can expect a lawsuit.
In Los Angeles in 1948, Julius "Jake" Berman hires private investigator J. J. "Jake" Gittes to catch his wife, Kitty, in the act of committing adultery. During the sting, Berman kills his rival, who also happens to be his business partner in a real estate development company. Gittes, not having known this, suddenly finds himself under scrutiny for his role in the possible crime, all of which centers around a wire recording that captured the illicit love meeting, the confrontation, and the killing of Mark Bodine. It calls into question if Berman knew and killed his partner to wrest control of the partnership, making it murder, or was an act of jealousy, which may qualify as "temporary insanity" and be permitted as a defense to a charge of murder.