In autumn 1379, the power of the crown is invested in John of Gaunt, and the kingdom is seething with discontent. The French are attacking the southern ports and peasants are planning a revolt organised by a mysterious leader who proclaims himself 'IRA DEI', the anger of God. In London, Gaunt's plans are plunged into chaos by a series of bloody murders. In desperation, Gaunt turns to Sir John Cranston to catch the assassin and recover a vanished king's ransom in gold. Cranston naturally calls on his ally, Brother Athelstan. They face threats and attack from the powerful as well as from the seedy underworld of medieval London, not to mention a chilling exorcism, as they try to bring order to their own lives and a subtle murderer to justice.