Kell is one of the last Antari—magicians who can travel between parallel Londons in different worlds. He serves as a royal messenger between Red London, his vibrant home, and the dying magic of Grey London. When he smuggles a dangerous artifact from a forbidden world, he crosses paths with Lila Bard, a cunning thief from Grey London who dreams of adventure. Together, they must outrun powerful enemies and prevent dark magic from destroying all the worlds.