1096, Byzantine Empire. Near the walls of Constantinople there is a camp of Frankish soldiers - participants in the First Crusade to the Holy Land. Their leaders are Godfrey of Bouillon, the founder of the mysterious secret society Priory of Sion (the predecessor of the Knights Templar), and his brother Baldwin, the future king of Jerusalem. On the eve of the arrival of the crusaders, a daring attempt was made to assassinate Emperor Alexios I Komnenos using a weapon unknown in Byzantium. The emperor's adviser entrusts the investigation to a former assassin, an experienced revealer of secrets, Demetrius Asciat, who immediately discovers that a mysterious monk is involved in the case. Trying to find the killer, Demetrius realizes with horror that there are some powerful forces behind the monk and that traitors must be looked for at the very top of Byzantine society...