On their way to Austria, the Marquise of Berkenfeld and her butler, Hortensius, have stopped in a small Tyrolean village be­cause the French army blocks their way. They ask Sulpice, sergeant of the 21st regiment, for permission to continue on­ward. Sulpice is joined by Marie, the “daughter” of the regi­ment, who was adopted as an orphaned child found on a battlefield. When Sulpice questions her about a young man she has been seen with, Marie explains that he is a local Tyrolean who saved her life. Soldiers of the 21st arrive with a prisoner: the same Tyrolean, Tonio, who has been looking for Marie. Tonio is taken away by the soldiers, but he escapes and returns to declare his love to Marie. Sulpice surprises them, and Marie admits that she can only marry a soldier from the 21st regiment.

The Marquise of Berkenfeld asks Sulpice for an escort to her castle. When he hears the name Berkenfeld, Sulpice remem­bers a letter he discovered near Marie when she was found, and the Marquise claims that Marie is the long-lost daughter of her sister. The Marquise is determined to give her niece a proper education. Tonio has enlisted in the army so that he can marry Marie, but she has to leave both her regiment and the man she loves.

Several months later, the Marquise has arranged a marriage between Marie and Scipion, nephew of the Duchess of Krakenthorp. Sulpice has joined the Marquise at the castle to recover from an injury. The Marquise gives Marie a singing lesson, but Marie slips in phrases of the regimental song, encouraged by Sulpice, and the Marquise loses her temper. Left alone, Marie hears soldiers marching in the distance and is delighted when the whole regiment files into the hall. Tonio asks for Marie’s hand, but the Marquise declares her niece engaged to another man. Alone with Sulpice, the Marquise confesses the truth about her situation.

The wedding party arrives, headed by the Duchess of Krakenthorp. Marie refuses to join, leaving the Marquise with an agonizing choice to make…