One summer morning while Aidan and Sarah are visiting their grandfather, they discover a secret compartment in his battered wooden desk. Inside is a yellowed envelope that contains a piece of very thin, almost translucent, white paper, on which, handwritten in black ink, are a series of seemingly random lines; among them are what appear to be fragments of letters, but not enough to make sense. At the bottom of the page is a verse about Peter Peter and a reference to a real hotel in London. As it happens, the family is about to embark on a trip to Europe, so the children decide that while in London, they will try to locate the hotel.
