The Marquise of Berkenfield is traveling in the Tyrols Fighting is raging, but Sergeant Sulpice of the Twenty-First Regiment says the regiment will restore order. Marie, the Regiment 'mascot,' enters and Sulpice questions her about a young man she has been seen with. She names Tonio, a Tyrolean, when Tonio is brought in as a prisoner.Marie sings the regimental song. Sulpice leads the soldiers off, taking Tonio with them, but Tonio runs back. She says she cannot marry without the approval of her 'fathers,' the soldiers of the regiment. The Marquise arrives and asks for an escort to her castle, Birkenfeld. Sulpice recognizes the name from a letter found with Marie as an infant. The Marquise recognizes Marie as her long-lost niece. Marie says farewell to the regiment. Tonio enters and announces he has enlisted to be near her.