England. Autumn, 1454.
Satan is trapped in the body of Sixth King Henry, rendering the monarch dumbstruck. A man most pious, his devotion to the gospels has made him an accidental prison for the King of Hell. Henry's queen, Margaret of Anjou, rules in his stead. But her command is tenuous, and the wolves are circling: Henry's capable and bitter relation, Richard Plantagenet, Duke of York, plots to seize the throne.
Meanwhile, world-weary London doctor James Stuart, his aged uncle Dr Matthew Stuart and James' restless apprentice Aristo Baros make the faraway journey to Northumbria, to the estate of Thomas Percy, Baron Egremont of Hexham. Haunting events have resurfaced, for in the doctors' pasts is the Devilmarker: an inscriber of ancient, talismanic symbols that exorcise evil spirits. Science and reason clash with sorcery and phenomena, and a summons to attend 'Mad King Henry' is impossible to resist - one that reveals a devilish reformation.