In the 1890s, Sergeant Major John Philip Sousa, the director of the United States Marine Band, leaves the United States Marine Corps after his enlistment expires to form his own band. He must do so because he is not paid enough by the Corps to provide for his wife Jennie (Ruth Hussey) and their three children. As a favor for his splendid service, he is allowed to take along Private Willie Little (Robert Wagner), who has invented and plays a new instrument, the "Sousaphone".