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 Clutter family is introduced. Perry and Dick are introduced. Perry and Dick's murder plot is made clear and carried out. The two are on the run, and the Kansas Bureau of Investigation is on the case.
Act II
Dewey and his agents interview everyone. Perry's boot prints are found at the crime scene. Then Floyd Wells comes forward and reveals that he told Dick about Herb Clutter and is sure he is the killer. After an exhaustive search Perry and Dick are traced to Las Vegas and arrested. Back in Kansas, they're confronted with Floyd's testimony. They eventually confess, and are tried, convicted, and sent to death row.
Act III
The pair spend five years on death row, going through a number of appeals and stays of execution. After their final appeal fails, Dick goes to the gallows first, forgiving his executioner. Perry is executed next, after speaking a few words against capital punishment. Years later, Dewey goes to the graveyard to visit the Clutters' graves and runs into Susan Kidwell, now a beautiful young woman in a hurry. Life has continued, and the debt has been paid.