On a September evening in 1931, John and Jack, two of the Caretakers of the Imaginarium Geographica, discover a plea for help on an ancient medieval parchment. It seems to have been written by their friend, Hugo Dyson! But when they rush to warn him, they find that Hugo has already been abducted by fierce creatures called the Un-Men, who have mistaken him for the third Caretaker, Charles. And in that moment, the world begins to change…

The Frontier which separates our world from the Archipelago of Dreams has fallen. Dark and terrible beasts roam throughout England. No one can be summoned from the Archipelago. And worse, their mentor and ally Bert seems to have forgotten them entirely!

The only hope of restoring order from the chaos lies on a forgotten island - where a time travel device left by Jules Verne must be used to race through history itself - from the Bronze Age, to the fall of Troy and the founding of the Silver Throne. And in that single night, John and Jack discover that the only way to save their friend and stop the chaos destroying the world is to solve a two-thousand year-old mystery: Who is the Cartographer?