In 18th century Paris, a mysterious coffin carrying the body of noblewoman Lia de Beaumont is discovered floating in the Seine River. Her younger brother, Charles d'Eon, newly knighted, suspects a link between her death and the sudden disappearances plaguing the city. To uncover the truth, d'Eon joins the secret police and uncovers a sinister plot involving the French and Russian nobility, orchestrated by a cult. As d'Eon delves deeper, it becomes evident that Lia's demise was a result of unraveling this conspiracy. Amidst a planned arrest of the Duke of Orléans, a treacherous betrayal leads to a brutal massacre by a demonic entity. While attempting a rescue, d'Eon is possessed by Lia's spirit, who guides him in defeating the malevolent force. Now infused with Lia's vengeful soul, d'Eon must forge alliances to confront the supernatural threat lurking within the nobility.