Le Chevalier D'Eon takes place in 18th century Paris, where a coffin carrying the body of Lia de Beaumont, a spy for King Louis XV, is found floating down the Seine River. Lia's younger brother Charles d'Eon, who has just been knighted, becomes entangled in a mystery surrounding her death when several disappearances occur throughout the city. As d'Eon joins the secret police to investigate, he uncovers a conspiracy involving members of the French and Russian nobility, fueled by a cult, and suspects that Lia may have been killed for uncovering the truth on a mission. D'Eon's investigation is complicated by supernatural events, including a demon attack on the secret police and his own possession by his sister's vengeful soul.