The Diamond as Big as the Ritz" is a fantastic example of the "Voyage and Return" Booker plot – it fits the mold to a T. In fact, Booker even cites this story as a key example of this pattern. So let's get to it already. In this initial stage, John, who loves rich people, eagerly anticipates spending a summer with the very wealthy Washingtons. He is a bit of an outsider to the world of the super-wealthy because of his own origins. Sure, the Ungers are affluent, but, as the opening passages tell us, not the kind of socially elite people that John hob-nobs with at St. Midas' prep.
John is both exhilarated by and uncomfortable with the world of the Washingtons. The fact that he keeps falling asleep his first night there shows us how truly overwhelmed he is at the opulence that Percy takes for granted. It's clear that he is an outsider in this strange world; it's not a place where he could ever feel at home, certainly. Kismine is part of the appeal of this new world.
If you didn't get the hints yet, it becomes increasingly clear that there's something wrong with this picture. John gets a real glimpse of Washington's darker side when he sees the prisoners in the ground – but even this doesn't totally faze him. It's not until he hears that his own life is in danger that he really gets worried.
This is the stage in which the hero is seriously threatened. In John's case, he is first threatened by Braddock's own men – he discovers them in the hall and is certain that they were on their way to kill him. Then, he is threatened by the air-strike pioneered by Braddock's escaped prisoners. Will John make it out alive?
John's escape was not just a literal escape from the doomed château, but also from the world of wealth he entered at the start of the story. Notice that he plans a return to Hades, not to St. Midas' prep – meaning that the school and the Washington estate were part of the same foreign world. In his description of the seven basic plots, Booker says that a fundamental question arises at the end of the "Voyage and Return": have the main characters been fundamentally changed, or was it all "just a dream"? John and his companions actually consider this very question at the conclusion – see "What's Up with the Ending?" for more on this note.