The book is based on the murder of Yazdgerd III, the last emperor of Sasanian Persia, who while being hard pressed by the Arabs on his western flank, fled to Marv where he was slain in a mill, in which he had been taking refuge.
The story begins with the Zoroastrian high priest (magus) of the Persian Empire, accompanied by the imperial army commander entering the mill to try the miller accused of murdering the emperor. The miller, his wife and his daughter, while trying to exculpate themselves, all express a different version of the same incident. As the story shifts, more questions come up than are answered.