The Marquise of Berkenfeld and her butler, Hortensius, are on their way to Austria. They pause in their journey because a skirmish has broken out. The Marquise learns that the French troops have retreated. She comments on the rude manners of the French people (""Pour une femme de mon nom""). The sergeant of the 21st regiment, Sulpice, assures everyone that his men will restore peace and order. He is joined by Marie, the "daughter" of the regiment. She is an orphan and was adopted by the soldiers. Sulpice questions her about a young man she has been seen with. She explains that he is Tonio, a Tyrolean who once saved her life. Troops of the 21st arrive with a prisoner. It is Tonio. He has been looking for Marie. She steps in to save him. He toasts his new friends, and Marie sings the regimental song (""Chacun le sait""). Tonio is ordered to follow the soldiers. He escapes and returns to declare his love to Marie. Sulpice surprises them. Marie must admit to Tonio that she can marry only a soldier of the 21st.