For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The Op arrives from San Francisco to help Donald Willsson expose the criminal activity in Personville, only to find out that Donald has been murdered. Being a stranger in a new city, the Op has his work cut out for him tracking down the murderer. To make things worse, no one wants to help him. In fact, the deeper the Op digs into the history of Personville, the greater the danger he faces.
Act II
The Op follows one lead after another until he finally figures out the identity of the murderer. But his job isn't even close to being done yet. The Op is on a mission to cleanse Personville not merely of Willsson's killer, but of every single one of its dishonest citizens. He sets the wheels in motion by turning one mobster against another, but then he hits a snag. The Op becomes involved in the murder of Dinah Brand, and he isn't even sure himself whether or not he's the murderer.
Act III
After a series of twists and turns, the Op figures out who killed Dinah (luckily, it wasn't him). The Op's plan to isolate the mobsters from each other ends up working: they either kill each off or they're sent off to jail. Personville has been purified of its criminals, but we're not quite sure how long this peace will last.