Warning: Contains spoilers
Private eye Jake Gittes (Jack Nicholson), a successful man with a nasty reputation specializing in marital cases, is hired by Mrs. Mulwray (Diane Ladd) to investigate the alleged infidelities of her husband Hollis Mulwray (Darrell Zwerling), chief engineer of the city's water supply. Jake and his two associates follow Mulwray as he spends long hours investigating reservoirs, walking dry river beds, and staring at the ocean. The city is in the midst of a drought and he has, as Jake puts it, "water on the brain". But he's also got a mistress, who Jake discovers and photographs, only to find that the photos have instantly found their way to the newspaper and that the real Evelyn Mulwray (Faye Dunaway) is suing him. Just as quickly, Hollis is dead, and Jake finds himself in the middle of something much more complicated than photographing an affair.
But in the convoluted reality that is Chinatown, how certain can Jake be that he's actually witnessed an affair? Viewed through a zoom lens and taken at face value as validation of the job he was hired to do, it certainly looks like evidence of an affair, and that's what Jake assumes. The work is easier that way. When Evelyn asks Jake what he used to do in his previous life as a cop in Chinatown, he answers, "as little as possible," and that philosophy is one he seems to have adapted to his current line of work. The problem is that this approach leads to Jake drawing concrete conclusions — often incorrectly — at multiple points in the investigation. In a lot of ways it's a scattershot approach — he's sometimes right, sometimes wrong — and with a little more due diligence he could have the thing wrapped up quickly, but then it wouldn't be much of a film, would it?
Despite the Occam's razor methodology of finding the simplest explanation, Jake manages to get needlessly drawn into a complicated web. He puts his nose where it doesn't belong (and has the bandage to prove it). He gets involved personally with the case. He fails to take his own advice to "let sleeping dogs lie." He tells his one operative of the need for a certain amount of finesse, but fails to use any himself. This does not necessarily make Jake a bad private eye, just a headstrong one with some contrarian tendencies, almost a bull in the china shop. Still, his is a results-oriented profession and Jake manages by sheer will to get results, proving that there is a method to his madness or perhaps that he's got more finesse than we realize. He certainly isn’t the first private eye in the world to solve the case despite his methods.
Few actors could pull off a role this complex as well as Nicholson, who as the protagonist serves as the film’s ballast. Initially he appears to be playing a standard film noir private eye, but as the film progresses, he begins adding layers. He laughs a raucous laugh at a racist joke, nearly comes to blows with a bank manager, and gets into a couple of fights, which is pretty much what we expect from the character. But, he also shows tenderness and humanity where needed. He harbors no ill will toward the people who seem to be making his job more difficult, telling Russ Yelburton (John Hillerman) that he’s more than willing to pass the whole thing off on a couple of big shots.
One of the delights of Chinatown is that in this web of mystery and intrigue, the audience is never given any more information than Jake has at that particular moment, nor are we given any hint as to how reliable such information is. We know only that he has it and where he got it. Since the entire film is done from a first-person perspective, we're given no indication of the various machinations going on behind Jake's back. In essence, we are put in the position of one of his operatives, trying desperately to piece the whole thing together. Eventually we do, largely without chunks of exposition explaining the complicated proceedings and at the end of the film not only does the entire thing hold water, but it makes sense. This is no small feat for a film noir done without voiceover.