Outside a Tyrolean village, a group of villagers, expecting to be overrun by the victorious Napoleonic army, is joined by the Marchioness of Berkenfield, whose journey has been interrupted by the fighting. There is relief when they learn that the French have withdrawn, but alarm when Sergeant Sulpice appears, much to his amusement, as his intentions are peaceful. He is joined by Marie, the orphan girl who had been brought up by the regiment since she was a baby and who has just been made the regiment's vivandiere.
As Sulpice is interrogating her about a strange young man she has been seen with, the soldiers drag him in; he is Tonio, a Tyrolean peasant, who has been found hanging round the camp. Marie saves him from instant execution as a spy by telling the soldiers how he had saved her from falling over a precipice. They immediately hail him as a brother, but as they are summoned by rollcall, Sulpice makes sure Tonio is not left alone with Marie, although she claims him as her prisoner and promises to keep an eye on him.
Tonio manages to give the Sulpice the slip and rejoins Marie, who explains that the regiment are her collective fathers. They confess their love and wander off.
The nervous marchioness explains to Suplice that she wishes to resume her interrupted journey to her castle of Berkenfield. The name reminds him of a former officer, Captain Robert, a name which, in turn, has strong associations for her. She explains that her sister had been married to the captain and their daughter lost. Suplice tells her that the child had been found on the battlefield and is alive and well and her upbringing has fitted her for her role as an heiress - a claim shattered by Marie's rough-and-ready military vocabulary when she learns that the lady is her aunt. The marchioness wishes to take Marie away with her.
Tonio has decided to join the regiment to be near his beloved. The soldiers, although ready to accept him as a recruit, are dubious about his wish to marry Marie, until he assures them that she loves him. They give their consent, only to learn that Marie must leave them. All express their sorrow.
A few months later, in the castle of Berkenfield. The marchioness has arranged a marriage for Marie with the Duke of Krakentorp and has summoned Sulpice to help her secure Marie's consent. The marchioness, who believes that Marie has lost her unladylike ways, gives her a singing lesson, but the presence of Sulpice causes her to abandon the sentimental ditty in favour of a rousing regimental song, which he joins in.
The marchioness takes Sulpice aside, and Marie is suddenly surrounded by the regiment, including Tonio, who has been promoted to officer for his courage. Marie sends the soldiers off with the steward to try the cellars, while she and Tonio try to persuade Sulpice to plead their cause with the marchioness. She, however, is unmoved, sending the lovers off in different directions. She admits to Sulpice that Marie is not her niece, but her illegitimate daughter. She has set up the grand marriage to provide Marie with the position and security she cannot legally give her. Sulpice is convinced that the marriage would be in Marie's best interest. When the dowager Duchess of Krakentrop, mother of the bridegroom, arrives with other guests, she is affronted to find the bride absent. Marie, who now knows the secret of her birth, embraces her mother and prepares to sign the contract, but the soldiers, anxious for their daughter's happiness, tell the guests that she has been their vivandiere. They are at first scandalised, then charmed by Marie's sincerity. The marchioness, touched by Marie's readiness to sacrifice herself, agrees to let her marry Tonio.
