Ten-year-old Leo's future and the fate of the extraordinary Whippet Hotel, where his father is the maintenance man, are at stake when a series of cryptic boxes leads Leo to hidden floors, strange puzzles, and unexpected alliances.

When Arianna and her stuffed bear arrive at the elegant Grand Hotel, where children are not allowed, the looks of horror that greet them gradually change to appreciation.

At Greenglass House, a smuggler's inn, twelve-year-old Milo, the innkeepers' adopted son, plans to spend his winter holidays relaxing but soon guests are arriving with strange stories about the house sending Milo and Meddy, the cook's daughter, on an adventure.