The Marquise asks Sulpice for an escort to her castle. When Sulpice hears the name Berkenfeld, he remembers a letter he found near the young Marie on the battlefield. The Marquise soon admits that she knew the girl's father. She says that Marie is the long-lost daughter of her sister. The child had been left in the care of the Marquise, but was lost. She is shocked by the girl's rough manners. She decides to take her niece to her castle and give her a proper education. Tonio has enlisted so that he can marry Marie (""Ah, mes amis""). But Marie has to leave both her regiment and the man she loves (""Il faut partir"").