In mid-18th century France, D'Eon de Beaumont earns his family's noble title, and a position in the court at Versailles, when his elder sister Lia's body turns up in a coffin floating down the river Seine. Despite the presence of his lovely fiancée Anna, a servant to Queen Marie, D'Eon eschews the court in favor of joining the secret police, so that he may investigate the bizarre circumstances surrounding the death of his sister, who was also a secret agent and accomplished swordswoman. What he discovers is a dark and treacherous underworld of secret organizations and occult activities which are indirectly traceable back to the Duke of Orleans, suggesting that the use of magic and quicksilver to turn people into “gargoyles” is part of some grand plot against the king. Though framed for murder, D'Eon discovers that he secretly has the support of both the King and Queen, who arrange allies in the form of the boy Robin, the roguish Durand, and the graying legendary fencer Teillagory. His strongest and most unpredictable ally, though, proves to be the wandering spirit of Lia, who has found a suitable vessel in D'Eon's own body. When the circumstances are right, blood scrolls down his sword, unleashing her and her vengeance.