The protagonist is twelve-year-old Milo Pine, the adopted son of the innkeepers in the smuggling town of Nagspeak. The establishment, called the Green Glass House, is located in a huge, dilapidated manor house that looks like it was hastily cobbled together from ill-matched houses collected from a dozen different cities. At the very beginning of the Christmas holidays, the inn is usually empty, so Milo hopes to have a good rest. But the silence of the first frosty night is broken by the insistent ringing of a bell. One by one, five guests check into the inn. The appearance of each of them is shrouded in some mystery, which turns out to be intertwined with the history of the Green Glass House. Milo is also puzzled by his chance discovery - a fragment of a navigational chart. And meeting Maddie, the cook's youngest daughter, spurred the young man to begin a detective game, the goal of which is to find out why all these people have arrived and where this strange map leads. He doesn't yet know what a difficult task awaits him, or what he will learn about the mysterious guests, the house, and himself.