Private investigator J.J. "Jake" Gittes is approached by a woman purporting to be Evelyn Mulwray, who hires him to investigate her husband, Hollis Mulwray, the chief engineer of the Los Angeles Department of Water and Power. Gittes follows Hollis and photographs him with a young woman, leading to a scandal when the images are published in the newspaper. Soon after, Gittes is confronted by the real Evelyn Mulwray, who threatens to sue him for defamation. As Gittes digs deeper, Hollis Mulwray is found dead in a reservoir, and the investigation uncovers a complex web of water supply corruption and deceit involving some of the city's most powerful figures.

Gittes' investigation leads him to discover covert land deals and a plot to control the city's water supply, suggesting that Hollis Mulwray was murdered because he stood in the way. The intrigue intensifies as Gittes becomes romantically involved with Evelyn, who harbors her own dark secrets. His pursuit of the truth puts him at odds with sinister forces, making him realize that the conspiracy reaches further than he initially imagined. Betrayal and moral ambiguity loom as Gittes navigates through bribery, deceit, and peril in his quest to uncover a shocking truth about the city's power dynamics and to protect the people unknowingly caught in the plot’s crossfire.