Three young men are entrusted with the Imaginarium Geographica, an atlas of fantastical places to which they travel in hopes of defeating the Winter King, whose bid for power is related to the First World War raging in the Real World.
Nine years after they came together to defeat the Winter King, John, Jack, and Charles return to the Archipelago of Dreams and face a new challenge involving the Lost Boys and giants.
With the Archipelago of Dreams in the hands of the Echthroi and the link to the Summer Country lost, the Grail child, Rose Dyson, the new Cartographer, Edmund McGee, and the Caretakers Emeritus seek to rebuild the Keep of Time but face a terrible new enemy who was once an ally.