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. There are no clues other than the left–behind book. In an effort to uncover the dead man's identity, one of the police constables, who fancies himself a portraitist, sketches a likeness to send to other police stations. It turns out there's a strong chance the man worked on poisoned gases for the British government after the Germans had used them at Ypres during the late war.

Scotland Yard dispatches Inspector Rutledge to confirm the ID and to find out why the man died in such mysterious circumstances. Rutledge begins his investigation, dealing with villagers who clearly have something to hide and trying to decipher if the death links back to the Great War. And what does the huge chalk sculpture of a pale horse of the Apocalypse have to do with the crime?