John and Jack are mystified when they discover a cryptic warning on a medieval manuscript—a warning that is not only addressed to them, but seems to have been written by their friend, Hugo Dyson. But before they can discover the origins of the book, Hugo walks through a door in time—and vanishes into the past.

In that moment, the world begins to change. Now, the Archipelago of Dreams and our world both suffer under the reign of the cruel and terrible Winter King. Dark beasts roam the countryside, and terror rules the land.

John and Jack must travel back in time—from the Bronze Age to the library in ancient Alexandria to the founding of the Silver Throne—to find the only thing that can save their friend and restore both words. The solution lies in the answer to a 2,000-year-old mystery: Who is the Cartographer?