In Roman Polanski‘s Chinatown, Ida Sessions impersonates Evelyn Mulwray for a few hours in order to hire private investigator, J. J. “Jake” Gittes. She wants him to survey the real Evelyn’s husband, Hollis I Mulwray, and find out if he’s having an affair with another woman. Jake takes the job, and sets out to find out what he finds out.
He tails Mulwray, who is the Los Angeles Department of Water and Power’s chief engineer, to a public meeting on the creation of a new reservoir. Mulwray publicly opposes the dam because a dam he created years earlier failed; killing many people. They plan to build the new dam on the same earth foundations as the failed one. So he wants no part of it.
Eventually, Jake catches Mulwray redhanded with another woman, and publishes pictures of their encounter in the daily newspaper. However, the next day, the real Evelyn Mulwray (and her lawyer) greets Jake. His revelation angers them, and worry about its affects on her reputation.
She tells Jake to expect a lawsuit, but it’s unclear what she can sue him for or, why she’d want to expose herself in this manner. The aforementioned encounter a) takes place in a public space, and b) Evelyn wouldn’t want to take her chances that Jake can’t find out the truth about the mistress’ identity. He could then exploit that fact to save his own ass.
Jake doesn’t accept someone set him up, and sets out to learn who set up Mulwray. Before he’s able to talk to Mulwray, he winds up dead in a freshwater reservoir. Things turn a bit crazy from there.
The real Evelyn Mulwray drops her lawsuit against Jake, and hires him to find out what he finds out. Jake’s investigation leads him to discover that the reservoir lets out thousands upon thousands of gallons of water at night. Water Department Security Chief, Claude Mulvihill and his lackey interrupts his discovery. The midget lackey slashes Jake’s left nostril as a warning of what will happen if he doesn’t drop the case.
Later, Ida Sessions calls Jake, but refuses to tell him who her employer is. She does offer a clue when she reveals that one of them appeared in that day’s obituaries. Jake has no clue who that’d be thus far, so he goes to meet the very pleasant Noah Cross. He used to be Mulwray’s business partner, and is Evelyn’s bastard father.
Noah finds out from their conversation that Evelyn hired Jake to find out what he find out. So Cross offers to double Evelyn’s money plus a bonus if he finds Mulwray’s now missing mistress.
Jake heads off to the Hall of Records, and he learns someone is buying huge tracts of land in the Northwest Valley; the very valley where the drought has now taken hold. Jake goes to see some of the property for himself, but local farmers, thinking he’s another water department thug looking to poison their water or blow up their storage tanks, attack him.
Jake realizes that Cross, and some of his associates at the Albacore Club, faked the drought. Distressed farmers then have no choice but sell their land for cheap. Evelyn picks Jake up from a farm, and as they head home, Jake reads the day’s obituaries. One of deceased has the same name as one of the people who just bought a lot of land in the Northwest Valley, BUT he bought the land a week after his death.
He was a resident of the Mar Vista Inn retirement home, so Jake and Evelyn bluff their way into a tour of the place. Here, Jake confirms his suspicions that Cross and others at the Albacore Club are purchasing land in the Northwest Valley with the identities of the elderly residents at Mar Vista Inn. Jake narrowly escapes the home when Cross’s thugs show up to kill him, but Evelyn snatches him up in her car, and drives them back to her place.
Back at her place, Jake bones the noir out of Evelyn, but it ends when Evelyn receives an urgent phone call, and she frantically rushes off. Jake tails her to the mysterious location where through one of the windows, he spots Mulwray’s missing mistress. He confronts Evelyn about the mistress in her car as she starts to drive off. He claims Evelyn is holding the girl against her will, but Evelyn confesses that the girl is actually her sister, Katherine.
The next day, Jake heads over to Ida Session’s place, but he finds her dead. He also finds police Lieutenant Escobar and his merry police squad hiding in her bathroom. Escobar wants answers because Mulwray has salt water in his lungs, but that can’t be if the guy drowned in a fresh water reservoir. Escobar believes Evelyn murdered her husband, and then moved the body. He also thinks Jake is unlawfully hiding evidence for her, and orders him to bring Evelyn to him or face arrest.
Jake goes to Evelyn’s house, but she’s packing to leave town. So, he runs off to Katherine’s house where he retrieves a pair of broken bifocals out of her salt water pond; believing them to be Mulwray’s. He arrives at Katherine’s place, and finds Evelyn finishing up packing. Jake pressures Evelyn about Katherine. She breaks, and tells him that Katherine is both her sister and daughter because her father raped her at the age of 15.
Evelyn also tells Jake that the bifocals he found in the water are not Mulwray’s but her father’s (Noah Cross if you’ve lost track in all the names). Jake now links Cross to Mulwray’s drowning/murder in the salt water pond at Mulwray’s house, so he tells Evelyn to go to her butler’s house in Chinatown, and wait for him.
Jake goes back to Evelyn’s house again, and calls Cross to tell him he has Katherine. When Cross arrives, Jake confronts Cross with what he has found out. Cross admits that he is the mastermind behind a plan to annex the Northwest Valley into the City of Los Angeles; That will allow him to develope the cheap land he bought, and then sell the residents in the area the water he owns.
Things go down hill between the two men when Jake shows Cross his broken bifocals he found in the saltwater pond that Mulwray died in. Mulvihill shows up again, and takes the glasses. They then force Jake to drive them all to Evelyn and Katherine in Chinatown, but when they show up, Lieutenant Escobar and his men are waiting to arrest Jake.
As this is going on, Evelyn refuses to let Cross near Katherine. Evelyn shoots Cross in the arm, and drives away with Katherine. Escobar and his men fire at the car. A bullet hits Evelyn in the back of her head, and it explodes out her left eye. She falls face first onto the car’s horn; dead.
Cross gets his daughter for himself, the farmer’s land for himself, and the dam for himself. The city annexes his land, and Cross becomes even more wealthy from the whole shady ordeal. The cops release Jake, and tell him to: ”Forget it, Jake. It’s Chinatown.”