Chartres, 1145. Ten years earlier a fire destroyed the town and the culprit has still not been traced. The Chevalier Galeran de Lesnevan, summoned by the Count of Blois to investigate a new mystery, uses the opportunity to visit his friend Audouard, a master glazier, in Chartres on the way. When a murdered body is discovered in the river, a series of events leads Galeran to believe that the perpetrator of the fire ten years before is still at large in the area. While hundreds of master craftsmen, artisans and workers painstakingly rebuild the cathedral for the glory of God, Galeran lives among them in an attempt to solve the mystery.