John T. Unger is a teenager who hails from Hades, Mississippi. An emotional parting with his parents sees John head of to St. Midas Preparatory School, a private school in Boston. He visits the homes of his wealthy friends during the summer amidst being mocked about his hometown. It is in one of these occasions that he accompanies the visibly rich Percy to spend the summer at their place.
On the train journey, Percy reveals to John that his father is by far the richest man in the world, owning a diamond the size of the Ritz. John’s doubts on this fade once he sees the classy vehicle that takes them to Percy’s home. Braddock T. Washington, Percy’s father, lives in a gigantic château that rests on a mountain which actually is one solid and flawless diamond. Braddock’s land is the only area that is not surveyed in the country with efforts to survey it prevented by dubious means executed by the Washington family.
Percy’s grandfather, Colonel Fitz-Norman Kulpepper Washington discovered the fortune just after the end of the civil war. He recruited ignorant Negroes to exploit the diamond and within no time he had become the richest man in the world and even traveled a lot. Since the diamond was in nearly immeasurable quantity, he realized that its discovery by the other people would devalue his fortune. He extracts enough diamonds to obtain enormous wealth and guards the rest at all costs. Braddock inherited the diamond mountain and his father’s slaves and also resolved to keep the secret. He achieves this by shooting down aircraft that fly over the area and imprisoning the pilots in an underground hole. One of these prisoners is brought out to teach Italian to his daughter but manages to escape to much chagrin to Braddock. Braddock sends his men to find the Italian and fourteen men are thought to be him are killed.
Meanwhile, John enjoys his summer at the Washington’s’ and ends up falling in love with Braddock’s daughter, Kismine with whom he even plans to elope. Matters take a turn when John discovers that all guests to the Washington’s are murdered with the aim to keep the family secret. He learns the killings from an accidental slip of the tongue from Kismine, and their relationship sours as John realizes that Kimberly deems this to be okay. Kismine manages to convince John to take her with him when he escapes. That same night, however, Braddock’s land is aerially attacked by fighter planes presumed to have been brought by the Italian escapee. John, Kismine and Jasmine, Kismine’s older sister escapes form the chateau with a few diamonds and watch the attack from the safety of the woods.
As dawn approaches, John beholds a rather strange sight. Braddock is seen at the top of the mountain with his slaves carrying a huge piece of diamond. He attempts to bribe God with the diamond and promises to cut the biggest diamond in the world for God if He clears the mess that is still happening. God presumably declines the offer, and with his defenses exhausted Braddock leads the remaining members of his family and two slaves into the mountain. Kismine explains to John that the mountain is wired to explode, and the diamond mountain blows up massively. The Washington family, slaves and the offensive aviators who have already landed on the mountain die in the explosion.
John and the two sisters walk away from the mountain. During a rest the next night, John discovers that Kismine accidentally grabbed the wrong bag, one containing rhinestones instead of diamonds. While John is deeply concerned, Kismine is not bothered since she thinks it will be exciting to be poor. Before they sleep for the night, they plan to settle in Hades, John’s home where Kismine hopes to be a washwoman.