War is raging in the Tyrolean Mountains. The Marquise of Berkenfield, who is traveling in the area along with her faithful butler, Hortensius, finds that her path is suddenly blockaded by the French Army. When the Marquise hears from a group of local villagers that the French troops have begun to move away, she remarks on the boorish ways of the French (“Pour une femme de mon nom”). When Sergeant Sulpice of the Twenty-First Regiment of the French Army arrives to calm nerves and assure everyone the regiment will restore order, Hortensius asks Sulpice to let the Marquise continue her journey. Joining Sulpice is a young woman named Marie. She is the “daughter” of the regiment which adopted her as an orphaned baby. Sulpice is happy to see Marie, (“Mais, qui vient? Tiens, Marie, notre fille”) but begins to question her about the comings and goings of a young man whom she has recently been seen with. She identifies him as Tonio, a Tyrolean from a nearby village. At that same moment, the aforementioned Tonio is brought in as a prisoner who has been seen prowling around the camp. Marie steps in to save him from the soldiers, explaining that this Tyrolean—though technically an enemy—once saved her life when she nearly fell while mountain climbing. The regiment changes its tune and instead of imprisoning him, makes a toast to Tonio. As the soldiers celebrate, Marie begins to sings the regimental song: (“Chacun le sait”). Sulpice leads the soldiers off, taking Tonio with them, but Tonio runs back to declare his love for Marie. She is thrilled, but quick to tell him that he must gain the approval of her “fathers”—the soldiers of the regiment who found her on the battlefield as an abandoned baby—if he wants to marry her.
The Marquise of Berkenfield, having waited long enough, asks Sulpice to accompany her and Hortensius back to her castle. Upon hearing the name Berkenfield, Sulpice immediately recognizes it from a letter found with Marie as an infant. It is soon discovered that the Marquise knew the girl’s father and says that Marie is her long-lost niece. When Marie was just a baby, she had been left in the care of the Marquise, but in the tumult of war, the baby was lost, never to be found. Appalled by Marie’s unconventional upbringing and complete lack of manners, the Marquise brings her newly rediscovered niece back to her castle to right the many years’ worth of wrongs, teach her proper etiquette, and give her a decent education. Tonio, who has just enlisted and joined the Twenty-First Regiment (“Ah, mes amis”), is devastated to learn that Marie is leaving the regiment forever.
Marie has been living in the Marquise’s castle for several months. In a conversation with Sulpice, who has joined the Marquise and Marie at the Berkenfield castle, the Marquise describes how she has arranged a marriage for Marie with a young man named Scipion, nephew of The Duchess of Krakenthorp. The Marquise has sought to expunge Marie’s military manners and transform her into a lady of fashion suitable for marriage. Marie has reluctantly agreed to the arrangement and Sulpice is asked to encourage her. During one of Marie’s singing lessons with the Marquise, the young lady in training, emboldened by Sulpice, adds in phrases of the regimental song. The unimpressed Marquise becomes angry and storms away (“Le jour naissait dans la bocage”). As Marie is left alone to reconcile her fate, she remarks on the pointless nature of opulence (“Par le rang et l’opulence”). Suddenly, she hears the familiar sound of soldiers marching in the distance and is overjoyed to see that the whole regiment has arrived at the castle. The soldiers file into the hall, and while the Marquise takes in the horrific sight, Tonio asks for Marie’s hand, declaring his love for her (“Pour me rapprocher de Marie”). The Marquise interrupts the happy occasion to declare that her niece is already engaged to be married. Marie scornfully returns to her chambers while Tonio and his fellow soldiers are ordered to leave. After everyone has left, the Marquise and Sulpice are left alone. In a moment of true vulnerability, the Marquise confesses that Marie is her own illegitimate daughter whom she abandoned in fear of disgrace.
Marie’s fiancé Scipion and a wedding party headed by The Duchess of Krakenthorp have been invited to the castle Berkenfield. Hortensius announces their arrival, but Marie refuses to leave her room to greet the guests. Sulpice joins Marie in her chambers and shares the secret the Marquise has just told him. The bewildered young woman decides she cannot go against her mother’s wishes and instead goes against her heart. During the final moments of the wedding ceremony, as Marie reaches out with pen in hand to sign the marriage contract, her regiment rushes in to rescue their “daughter” (“Au secours de notre fille”). When the regiment reveals that Marie grew up as their canteen girl, the noble wedding guests become outraged. Their opinions soon shift, however, when Marie sings of her gratitude towards the soldiers (“Quand le destin, au milieu de la guerre”). Deeply moved by Marie’s love for her “fathers,” the Marquise openly declares that she is Marie’s real mother and gives her blessing to Marie and Tonio. All join in rejoicing this happy occasion (“Salut à la France”).