After discovering his older sister has been inexplicably murdered, D’Eon de Beaumont joins the French secret police–which has been investigating the disappearances of numerous young women–in hopes of solving the mystery of her death.

After being possessed by his sister’s soul, D’Eon awakens in the royal palace remembering nothing about the incident. Soon he is pursued by French police, and is unexpectedly aided by a young swordsman named Durand.

King Louis XV gives the Four Musketeers a mission, and D’Eon asks Queen Marie why his sister’s soul has not passed on.

Following the Empress’ death, Pyotr has become Emperor of Russia. Ekaterina, the woman who warned the Four Musketeers, steals the throne back from him with their help. The Four Musketeers continue to battle members of the Revolutionary Brethren.

D’Eon’s battle against Vorontsov is finally finished, with Lia dealing the final blow. The Four Musketeers follow Robespierre to London.

The Four Musketeers meet with the French Ambassador to England, and they attend a party thrown by the English king and queen intending to steal important documents held by the British Undersecretary of Foreign Affairs.