General Armand de Montriveau, a war hero, is enamored of Duchess Antoinette de Langeais, a coquettish, married noblewoman who invites him to a ball but ultimately refuses his sexual advances and then disappears. Assisted by the powerful group known as The Thirteen, who subscribe to an occult form of freemasonry, General Montriveau finds the duchess in a Spanish monastery of Discalced Carmelites under the name of Sister Theresa.