La 4ème de couv. indique : "Kell is one of the last Travelers--magicians with a rare, coveted ability to travel between parallel universes--as such, he can choose where he lands. There's Grey London, dirty and boring, without any magic, ruled by a mad King George. Then there's Red London, where life and magic are revered, and the Maresh Dynasty presides over a flourishing empire. White London, ruled by whoever has murdered their way to the throne--a place where people fight to control magic, and the magic fights back, draining the city to its very bones. And once upon a time, there was Black London ..."
