Welcome to Grey London, dirty and boring, without any magic, with one mad king - George III. Then there is Red London, where life and magic are revered, and White London, a city slowly being drained through magical war down to its very bones. And once upon a time, there was Black London ... but no one speaks of that now.

Officially, Kell is the Red Traveler - one of the last magicians who can travel between worlds - acting as ambassador and messenger between the Londons, in service of the Maresh empire. Unofficially, he's a smuggler, which is a dangerous hobby for him to have - as proved when Kell stumbles into a setup with a forbidden token from Black London. Fleeing into Grey London, Kell runs afoul of Delilah Bard, a cutpurse with lofty aspirations, who first robs him, then saves him from a dangerous enemy, and then forces Kell to spirit her to another world for a proper adventure. But perilous magic is afoot, and treachery lurks at every turn. To save all of the world, they'll first need to stay alive.