A coffin is discovered floating down the Seine River in 18th-century Paris. It transports the body of noblewoman Lia de Beaumont, a spy for King Louis XV whose younger brother Charles d'Eon was just knighted. When other disappearances occur in Paris, the young knight suspects they are related to his sister's murder. D'Eon joins the secret police to investigate the occurrences in the hopes of finding her killers. Following the evidence, they piece together that the disappearances may be the result of a plot between members of the French and Russian elite, fueled by a cult. D'Eon comes to the conclusion that Lia may have discovered the truth while on a mission and was as a result slain. The secret police are scheduled to arrest people that night.