John T. Unger is a sixteen-year-old boy from an affluent family in Hades, Mississippi on his way to St. Midas' preparatory school in Boston, the most exclusive and expensive prep school in the world. There, he hobnobs with the wealthy and meets another student named Percy Washington. Percy invites John to spend the Summer with his family "out West," and John, who loves being with the super-wealthy, agrees.
On the train wide West, Percy reveals that his father is the richest man in the world. He has a diamond the size of the Ritz-Carlton Hotel. John soon discovers that Percy is telling the truth. Percy's father, Braddock T. Washington, has built an enormous château on a mountain that is literally one solid, flawless diamond. The diamond sits in the middle of five square miles in the woods of Montana – the only part of the country that has never been surveyed. The United States doesn't know that these five square miles exist at all, and the Washingtons plan on keeping it that way.
As a guest at the enormous estate, John soon learns the history of the Washington family. Braddock's father, Colonel Fitz-Norman Culpepper Washington, discovered the diamond mountain shortly after the Civil War had ended. He realized he was in a bit of a pickle: by owning the diamond, he was the richest man in the world. But if anyone ever discovered the diamond, it would lose its value, as diamonds would no longer be a rare gem. (The diamond is so large that it would essentially flood the market.) So he decided to hide its existence, at all costs. He brought slaves out from his home in Virginia after convincing them that the South had won the Civil War and that slavery was still legal.
Braddock inherited the diamond and a large number of slaves from his father, as well as the mission of keeping the diamond hidden. Today, he shoots down any planes that fly overhead and keeps the aviators prisoner in a hole in the ground (though he treats and feeds them well). We learn that, recently, an Italian prisoner escaped, much to Braddock's distress. He sent men after the fugitive, but he's not sure if any one of the men they killed was indeed the wanted man.
John seems to have no issues with the Washingtons' system; he has a grand old time spending the summer enjoying the Washingtons' lavish wealth, great food, and endless stream of servants. He also falls in love with Kismine, Percy's younger sister. The two of them make adorable plans to get married next summer. It's all going just swimmingly until Kismine reveals, accidentally, that her father never allows guests to leave their estate. Instead, he murders them in their sleep, in order to safeguard his diamond. Kismine doesn't seem to think there's anything wrong with this. She thinks that it's only right that she and her siblings should get as much pleasure as possible out of life.
Naturally, John is upset. He and Kismine make plans to run away and elope. Before they can, however, they find themselves under a nighttime attack from about a dozen fighter planes (presumably brought by the Italian who escaped from Braddock's prison). John, Kismine, and her older sister Jasmine escape to the woods. As they leave the château, John advises Jasmine to take a pocketful of jewels with her, which she does.
From a safe hiding spot in the woods, John watches a strange sight. Braddock has two of his slaves bring a giant diamond to the top of the mountain. As the fight planes continue to bomb his estate, he holds the diamond up to the sky and attempts to bribe God. He declares that, if God just clears up this whole mess, he will build Him a giant cathedral out of the biggest diamond the world has ever seen. God declines.
So Braddock leads his family into the mountain which, Kismine explains to John, is wired to blow. The whole place goes up in a massive explosion, killing the rest of the Washington family (Percy, Braddock, and Braddock's wife) as well as all the aviators who had landed on the mountain. John, Kismine, and Jasmine, however, are safe in the woods.
After the fireworks are over, the three make their way away from the mountain. When they stop to rest for the next night, John asks Kismine to bring out the jewels she took as they were escaping. Unfortunately, Kismine went for the wrong drawer and accidentally grabbed a handful of rhinestones. This doesn't bother her, however, as she was bored of diamonds and, beside, she thinks being poor for a change might be exciting. The three of them make plans to go live in Hades, where Kismine looks forward to working as a washerwoman. They fall asleep for the night.