The Clutter family, parents Herb and Bonnie and teens Kenyon and Nancy, are a happy, prosperous, civic-minded, church-going family living on their farm in Holcomb, Kansas. The narrator follows the Clutters through the ordinary events… of their last day on earth.
Perry Smith and Dick Hickock, a.k.a. the killers, slaughter them late that night.
Perry and Dick flee after the murders and make it to Mexico.
Kansas lawman Alvin Dewey and his four special agents lead the manhunt for the killer or killers. They're eventually led to Perry and Dick by Floyd Wells, an inmate serving time for theft. Floyd used to be Dick's cellmate and employee of Herb Clutter, and Floyd told Dick all about working for the rich farmer. (Floyd didn't bother to correct Dick when Dick assumed that there was a big safe full of money in Herb Clutter's office; and he didn't correct him when Dick started to make plans to steal from the Clutters and leave no witnesses.)
After carrying out their plans, in what can arguably be called the dumbest criminal move ever, Perry and Dick return to Kansas from a brief flight to Mexico and other venues, to pass some bad checks for travel money. Dick uses his own name to sign the checks.
The police finally arrest our villains in Las Vegas, after Floyd Wells sings like a canary. An eventual confession by Perry and Dick gives us a blow-by-blow account of the brutal murder of the Clutter family.
And we mean brutal.
They're tried, convicted of murder after 40 minutes of jury deliberation, and sent to Death Row.
Along the way, we're treated to retellings, through letters and flashbacks, of Perry's dreadful, awful childhood—including parental abuse, alcoholism, sibling suicide, and not being permitted to go to school (gasp!). We learn about Dick's childhood—not a bad one, actually, although he never seemed to be really attached to the idea of settling down. Never seemed to be attached to anything, really.
We get an in-depth view of both killers' psychological and sexual problems as the author tries to figure out what made them into the cold-blooded killers of the title. Five years after their conviction, both men are executed by hanging and the community tries to recover from the trauma.