When his apprentice is wounded in a terrorist bombing, Master Alchemist Castamar wants answers. The country stands on the verge of an industrial age ushered in by the mysterious energetic substance known as Vril. But revolutionary technology brings revolutionary ideas, and someone is using it to bring the kingdom to its knees.

Cast's search leads him to the Heron Kings, folk heroes of old thought lost to history and legend. But some legends die hard. Ellyx, the band's top thief and forger, joins Cast to hunt the culprits down and plumb the depths of his past for clues. When they learn that fanatics led by a shadowy Duchess have devised a terrifying variant of Vril that can decimate entire cities, Cast and the last of the Heron Kings will race to prevent a war of mass destruction.