In London in 1811, actress Rachel York is found murdered on the altar steps of a church near Westminster Abbey. A pistol and a witness place Sebastian St. Cyr at the scene, forcing the former cavalry officer to flee before the authorities can hang him for a crime he did not commit.
Sebastian turns to his former lover Kat Boleyn, surgeon Paul Gibson and street boy Tom while retracing Rachel's last movements through theatres, government offices and aristocratic houses. The evidence reveals that her death is tied to espionage and a secret capable of damaging both the monarchy and men determined to preserve their power.