In 18th century Paris, a coffin is found floating down the Seine River. It carries the corpse of noblewoman Lia de Beaumont: a spy of King Louis XV, and whose younger brother Charles d'Eon has just been knighted. When several disappearances occur throughout Paris, the young knight believes that they are somehow connected to his sister's death. Hoping to find her killers, d'Eon joins the secret police to investigate t...