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.
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.
When, in 1931, there is a breach between this world and the Archipelago of Dreams, John and Jack, two of the Caretakers of the Imaginarium Geographica, must race through history using a time travel device left by Jules Verne, and discover the identity ofthe Cartographer.
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.
The Caretakers, John, Jack, and Charles, return home after seven years only to find they must travel through a forgotten door in the destroyed Keep of Time to find the Dragon's Apprentice and restore Time--a quest that has serious consequences whether they win or lose.
The Winter King's Shadow, having gained control of the doors from the Keep of Time and of an army of Dragon Shadows, plans to use the turmoil of World War II to conquer worlds, but all Caretakers, past and present, come together to stop him, using some unlikely weapons.