Kell, the main character, is one of the last Travelers-magicians. He is able to travel between parallel universes that are connected by one city: London. There's Grey London, dirty and boring, and without any magic. Red London, where life and magic are revered. White London-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. But no one speaks of that now.
Officially, Kell is the Red Traveler, and he carries monthly correspondences between the royals of each London. Unofficially, Kell is a smuggler. It's a defiant hobby with dangerous consequences, which Kell is now seeing firsthand.
Fleeing into Grey London, Kell runs into Delilah Bard, a cut-purse with lofty aspirations. She first robs him, then saves him from a deadly enemy, and finally forces Kell to spirit her to another world for a proper adventure.
Now perilous magic is afoot, and treachery lurks at every turn. To save all of the worlds, they'll first need to stay alive. (Adapted from the publisher.)