In 1755, at the Palace of Versailles, a general of the French royal guard despairs of never having a son to carry on his military legacy. When his sixth daughter is born, he makes a fateful decision: he names her Oscar and raises her as a boy, training her from infancy in swordsmanship, horsemanship, and the arts of combat and command. As a young woman, Oscar Francois de Jarjayes becomes the commander of the Royal Guard, responsible for the safety of the young and flighty Queen Marie Antoinette. Oscar lives a dual existence, navigating the decadent opulence of the court of Versailles while possessing a keen awareness of the growing poverty and unrest festering among the common people of France. Torn between her duty to the crown and her innate sense of justice, she is also a woman forced to suppress her own heart and desires, living as a man in a world of rigid social hierarchy and romantic intrigue.

The narrative unfolds against the backdrop of the years leading up to the French Revolution. At court, Oscar is a striking and admired figure, but her deepest bond is with her childhood companion and servant, Andre Grandier. The grandson of her nanny, Andre is a commoner who has shared Oscar's life since childhood, acting as her sparring partner, attendant, and closest confidant. As they mature, their relationship deepens into a profound, unspoken love. A central narrative arc involves the passionate and scandalous affair between Marie Antoinette and the Swedish Count Axel von Fersen. Oscar is drawn into this affair, forced to protect the queen’s reputation while secretly sympathizing with the count's departure to fight in the American War of Independence. The queen’s subsequent spiral into lavish spending on jewelry and fashion to fill the void left by von Fersen, combined with the Affair of the Diamond Necklace and the manipulative influence of the Duchess of Polignac, fuels the public’s growing hatred of the monarchy and deepens France’s financial crisis.

As revolutionary fervor intensifies, Oscar can no longer ignore the suffering of the French people. She cuts her long hair, symbolically shedding her aristocratic identity, and leaves the Royal Guard to join the commoner-led French Guards. Andre, whose vision has been progressively failing in a secret injury, stands by her side. On July 14, 1789, during the storming of the Bastille, Oscar and Andre lead the insurgents into battle. Andre is fatally wounded protecting Oscar, and moments later, Oscar herself is shot and killed, falling on the battlefield as the Bastille falls, signaling the end of the absolute monarchy. Following the revolution, Marie Antoinette is captured, imprisoned, and ultimately sentenced to death by guillotine, bringing the era of the Ancien Regime to a bloody close.