Told largely through flashback, Perry Smith, Robert Blake, and Dick Hickock, Scott Wilson, meet in rural Kansas in late 1959. Together, they devise a plan to invade the farm home of the wealthy Herbert Clutter, John McLiam, family, convinced that the patriarch keeps a large stash of cash behind a wall safe.
The pair break into the house in the middle of the night but are unable to locate a safe, as Herbert uses checks for both personal and farm transactions. In order to leave no witnesses to their failed robbery, Smith and Hickock murder the entire Clutter family: Herbert Clutter, Bonnie Clutter, Ruth Storey, and Nancy Clutter, [Brenda Currin](/actor/brenda-currin, along with their young son Kenyon. The bodies are discovered the next day, and investigations by the Finney County Sheriff and Kansas Bureau of Investigation (KBI) are immediately launched.
Based on a tip by a former cellmate of Hickock, the two men become the primary suspects for the crime. The pair elude law enforcement by heading to Florida, traveling southwest across the country, and eventually crossing into Mexico. After two weeks there they return to the United States broke, and head for Las Vegas, hoping to win money gambling. Shortly after their arrival, Smith and Hickock are arrested for driving a stolen car, violating parole, and passing bad checks.
The Las Vegas Police Department and the KBI later separately interrogate the two men about the Clutter case. Both Smith and Hickock admit to passing bad checks, but deny knowing anything about the murders. The KBI attempts to scare the men into confessing, claiming that they left a witness behind who can testify against them, but this attempt fails. Next, the KBI confront the two with evidence, such as photos of bloody footprints matching each of their footwear. Finally, Hickock relents, confessing that he was present, but that Smith carried out the murders. He begs for immunity from the death penalty. After Smith learns that Hickock has cracked, he confesses that he committed all four frenzied killings, but maintains that Hickock was present as an active accomplice.
Both Smith and Hickock are charged with first-degree murder, found guilty on all counts, and sentenced to hang. After losing multiple appeals stretching over five years, with two reaching the United States Supreme Court, both men are put to death in front of witnesses as prescribed by law.