Because his family lives in Paris, on Epiphany, Oliver gets to wear the golden paper crown, because he found the key (a rather odd key, at that). Though he is old enough to have noticed that his mother always ensures that. And when he sees his reflection, he says, aloud, that he is the King in the Window.
He didn't know what he was getting into. But the people looking for a new King in the Window think why not? They haven't had much luck in getting a good one, and Oliver was no worse than another random selection.
A story ensues. It involves mirrors, crystals, windows, wraiths, the wittiest woman Oliver's father knows, queens, shadow pictures, in-line skating, the One with None (None what? Well, that would be a spoiler.), rhetoric, French history, the inventor of mayonnaise, a neighbor named Neige, and perfect mirrors.