The story follows John T. Unger, who is invited by his wealthy classmate Percy Washington to spend the summer at his home in Montana. During the train ride, Percy boasts that his father Braddock Washington is the richest man in the world and owns a diamond bigger than the Ritz-Carlton Hotel. When Unger arrives, he discovers the land is unsurveyed and learns the bizarre truth of Percy's claims - Braddock Washington had found a diamond the size of a mountain and built an extravagant estate around it, imprisoning anyone who discovered his property.