The dramatic arc of Chinatown is seen in Jake's discovery of political and personal manipulations.
Initially hired to spy on Hollis Mulwray, Jake is pulled into a complicated political and emotional labyrinth. He believes he was hired by Mrs. Mulwray. However, upon realizing that he had been "set up," Jake has to parse through who is legitimate and who possesses an ulterior motive. As an outsider not working with the police department or any other institutional agency, he fights to maintain his professional and financial autonomy. The film's arc develops as he tries to sort out the complicated relationship between the Mulwrays, Noah Cross, the California water supply, and the desire for wealth and power. The dramatic arc of the film takes him through corrupt members of the water and police departments, the power of the wealthy, and the disenfranchisement of the poor.
The film's arc develops as Jake gains more insight into this maze. As Jake discovers the elements that govern this world, so do we. He discovers the conspiracy regarding the public control of water and its wrestling of ownership of land from farmers to wealthy real estate magnates. Jake also discovers the tangled emotional world of Evelyn Mulwray. He struggles to understand the relationship she has with her husband, her father, and her daughter/sister. The arc of the film progresses as Jake gains more insight into all of these political and emotional elements.
It is not surprising that the film's dramatic arc corresponds with Jake's own emotional development. From being motivated by money to the desire to know the truth, Jake progresses into wanting to do the right thing for Evelyn and Katherine. Jake's motivation to doing right is met with the futility of good intentions in the form of overwhelming political power and personal manipulation.