Set in 18th century Paris, the story follows D'Eon de Beaumont, a newly knighted man who seeks to uncover the truth behind his sister Lia's mysterious death.
His investigation leads him to believe her murder is connected to a series of disappearances plaguing the city.
Joining the secret police, D'Eon delves into a web of conspiracies involving French and Russian nobles and a strange cult.
As he gets closer to the truth, a supernatural element emerges, revealing a deeper, more dangerous plot than he initially imagined.
D'Eon, now grappling with a supernatural connection to his sister, must find allies and confront the powerful forces at play to avenge Lia and protect France.