Late on a spring night in 1920, five boys cross the Yorkshire dales to the ruins of Fountains Abbey, intent on raising the devil. Instead, they stumble over the devil himself, sitting there watching them. Terrified, they run for their lives, leaving behind a book on alchemy stolen from their schoolmaster. The next morning, a body is discovered in the cloisters of the abbey—a man swathed in a hooded cloak and wearing a gas mask.

Scotland Yard dispatches Inspector Rutledge to find out who the man was and why he died in such mysterious circumstances. But the villagers clearly have something to hide. And what does the huge chalk sculpture of a pale horse of the Apocalypse have to do with the crime?