An unusual murder brings together three strangers, John, Jack, and Charles, on a rainy night in London during the first World War. An eccentric little man called Bert tells them that they are now the caretakers of the Imaginarium Geographica -- an atlas of all the lands that have ever existed in myth and legend, fable and fairy tale. These lands, Bert claims, can be traveled to in his ship the Indigo Dragon, one of only seven vessels that is able to cross the Frontier between worlds into the Archipelago of Dreams.

Pursued by strange and terrifying creatures, the companions flee London aboard the Dragonship. Traveling to the very realm of the imagination itself, they must learn to overcome their fears and trust in one another if they are to defeat the dark forces that threaten the destiny of two worlds.