We start off by meeting "the kid," a fourteen year-old who decides to run away from home and go it alone in the Old West. Sounds like a great idea, right? Wrong.
The thing is that he's walking into one of the most brutal and violent cultures in the history of the planet. To his credit, he does pretty well to survive his teenage years despite getting stabbed and shot on several occasions. Eventually, he's so hard up for money and clothes that he joins a militia that plans on going into Mexico to establish white American dominance. Yeah, we're sure that'll go over well with the Mexican people.
The militia's trip to Mexico is doomed from the start. They're barely inside the country before they're wiped out by a group of Apache warriors. Next thing you know, the kid is a prison slave. Luckily for him, he's bought out of prison by a man named Judge Holden who's helping another man named Glanton put together a group to hunt down Apaches and take their scalps for a government bounty. Cheerful. The kid falls in with this crowd and starts killing Apaches in no time. The problem is that once Glanton's crew gets a taste for killing, they turn into a mob of bullies who take whatever they want whenever they want it.
It's not long before Glanton's crew gets itself on Mexico's "Most Wanted" list, even as the Mexican government continues to pay them for Apache scalps. It basically gets to the point where the Mexicans decide that they don't know who they want to kill more: the Apaches or Glanton's men. In any case, a group of Yuma Aboriginals eventually solve this problem for the Mexicans by killing Glanton and nearly all his men.
The only people who survive the Yuma attack are the kid, Judge Holden, and a few other dudes. And worse yet, Judge Holden has decided he wants to kill the kid for being too much of a weakling. The judge believes that all the weak people in the world need to be cleared away so the strong can survive. But the kid escapes.
It's not until ten years after the fall of Glanton's men that the judge and the kid run into each other in a bar. The kid wanders out to an outhouse and finds the judge inside waiting for him. The judge pulls him inside and locks the door. We never find out what happens after that. We just know that whatever it is, it's bad. The book ends with Judge Holden dancing a jig back inside the tavern and telling people he'll never die. Just pat yourself on the back why don't you, Mr. Judge?