The narrative opens on a brisk morning in March 1718, introducing the Baron de Valef and Captain Roquefinette as they ride toward a duel at Port Maillot on behalf of the young Chevalier d'Harmental. The duel brings together fiery aristocrats including the Marquis de Lafare, the Comte de Fargy, and the youthful Chevalier de Ravanne, resulting in crossed blades and mutual respect among the combatants. Afterward, D'Harmental receives a cold rejection letter from his mistress, Madame d'Averne, which leaves him shattered and vulnerable to new ambitions. Promptly summoned by a mysterious masked figure wearing a violet ribbon, he attends a masked ball at the Palais Royal and is led blindfolded to a secret meeting place.
There he is presented to the ambitious Duchesse de Maine, Cardinal de Polignac, and other conspirators working to undermine the regent and support the cause of Spain. Entangled in this web of political intrigue, D'Harmental agrees to lead a daring plot to seize the regent. He enlists the formidable Captain Roquefinette, establishing a crimson ribbon signal at his garret window to coordinate their dangerous movements.
Meanwhile, D'Harmental catches glimpses of his charming neighbor Bathilde across the narrow street, experiencing a quiet romance that contrasts sharply with the brewing violence of the conspiracy. The plot finally culminates in a chaotic nighttime ambush in the Rue des Bons Enfants, where the regent escapes through an astonishing feat of agility over the rooftops, leaving the conspirators thwarted. In the aftermath, the captain advises D'Harmental to accept the failure philosophically, while the abbe and other plotters reckon with the dangerous reality of their clandestine enterprise.
