July 1241. Christendom cowers in terror before the threat of a Mongol invasion. The swift-moving, invincible cavalry of Genghis Khan has smashed the proud, steel-clad knights of Russia, Poland and Hungary - and now Austria lies directly in their path. At a skirmish outside the walls of Vienna, German knights capture a squad of Mongol scouts, and are astonished to discover one of their number is an Englishman - a Templar knight - who has been riding with these Devil's horsemen for more than 20 years. Interrogator Father Ivo of Narbonne is summoned to Vienna Castle to draw the truth from the imprisoned Englishman before his impending trial, to find out why he abandoned his Faith, his Brethren and his homeland to become. . .a traitor to Christendom.