Isabelle Rostain starts as a 15-year-old Parisian noble, the sheltered daughter of aristocrats Leon and Marie during the Franco-Prussian War. Her brother serves as a French army officer, while her sister pursues a romance with their commoner piano teacher, who aids Paris's defense against the Prussian siege. Initially carefree within her privileged world, Isabelle confronts the harsh realities of war and societal suffering as the conflict escalates.

When Paris falls under siege, Isabelle escapes to Versailles with Jean, her devoted childhood friend who carries a secret affection for her. Faced with the city's desperation, she volunteers for a perilous espionage mission to London seeking aid for Paris. To evade detection, she cuts her hair and adopts a male identity strictly for subterfuge—a practical disguise, not an exploration of self.

Isabelle's journey transforms her from detached nobility into an active resistance participant. Her evolution mirrors her family's shift from upholding aristocratic tradition to defending wider societal ideals. The conflict exacts a heavy toll; Isabelle sees the war claim family members and loved ones in a tragic conclusion.

Official records indicate Isabelle survives the war only to endure years of hardship. She faces profound trauma, poverty, and displacement on the streets of Paris, struggling to rebuild her life amidst the war's deep personal and societal devastation.