In 1890, noted composer and Marine sergeant major John Philip Sousa, who has been the leader of the famed Marine Corps Band in Washington, D.C. for twelve years, remains frustrated that only his marches, not his ballads, are embraced by the public. Sousa is supported by his adoring wife Jennie and idolized by musicians, including young Marine private Willie Little, who has modified a tuba and named it the "Sousaphone" in his honor. After Willie gets into a fistfight, commander Maj. George Porter Houston allows him to join Sousa's band so that Sousa can keep an eye on him, and Sousa becomes attached to the enthusiastic young man. One evening, Willie persuades Sousa to visit a theater where his ballads are to be sung, but the "theater" turns out to be a vaudeville house in which Willie's girl friend, Lily Becker, is featured as a dancer. When the theater is raided because of the dancers' skimpy costumes, Sousa, Willie and Lily escape to the Sousa home, where Willie convinces Sousa and Jennie to listen to Lily, an aspiring singer, perform. Sousa is bemused by Lily's frenetic style, but encourages her to take singing lessons. Later, Sousa, who has served under five Presidents, introduces his new march, "Semper Fidelis," at a reception hosted by President Benjamin Harrison and his wife, much to Harrison's pleasure. Sousa is given a special award for the song, which is the only composition to have received official recognition from the United States government. Soon after, Sousa informs a disappointed Houston that he will not be re-enlisting in the Marines and intends to form his own band. Houston pleads with Sousa to stay, but the composer is determined, as he has three children to support and has received only $105 in royalties for his marches. At Sousa's request, Willie is released from the Marines and accompanies him to New York, where he holds auditions. Sousa warns the musicians that he will insist on strict military discipline and will not allow wives to accompany them on tour. Willie fibs to Lily, telling her that Sousa wants to audition her for the band, but when she discovers that he has already hired opera singer Estelle Liebling, she is furious. Soon, Sousa has assembled a group of superlative musicians, and their performance at the Chicago's World Fair is a success. As the band tours the world, Sousa earns the nickname "The March King," while back in the U.S., Lily continues to study with the singing teacher he selected for her. One day, Willie and Lily share a passionate kiss, and Lily, disturbed by her "unladylike" response, asks Jennie for advice. Jennie counsels Lily to kiss Willie back, and also informs her that Sousa plans to include her in the band's next tour. Lily is upset, for she had hoped to marry Willie but does not want to break Sousa's rules. Jennie laughingly tells Lily that what Sousa does not know will not hurt him, and soon after, Lily and Willie marry in secret. After the band plays at the Dancing Masters Convention in Atlantic City, Sousa is informed that their upcoming engagement at the Cotton States Exposition in Atlanta has been canceled because marching bands supposedly do not draw big enough crowds. Disgruntled, Sousa shepherds his group onto a train bound for Atlanta, and during the journey, sees Willie sneak into Lily's drawing room. Jennie calms down the outraged Sousa by informing him of their marriage, and Sousa agrees to keep his knowledge secret. Sousa's band is a hit at the exposition, and their happy times continue over the next few years. Sousa often seeks new ways to entertain his audience, and the elaborate numbers he stages around Lily are well-received. One day, however, the band learns that the Maine has been sunk, and Willie re-enlists in the Marines. Sousa also re-enlists, but an attack of typhoid fever prevents him from serving. Sousa recovers and while working on the music for a comic opera, composes a new march, named "The Stars and Stripes Forever," and predicts that it will be his most popular composition. Later, during rehearsals of the show, which features Lily, Jennie receives a letter from Willie, stating that he has been wounded and may lose a leg. Sousa comforts Lily and admits his knowledge of their marriage. After the war's end, groups of wounded men return home, and in Brooklyn, where Willie is hospitalized, Lily escorts him to an evening's entertainment at the recreation hall. Willie is thrilled to see that Sousa's band is to perform and is deeply touched when Sousa calls him to the stand, where his sousaphone is waiting. Willie, who has lost his left leg, joins his comrades on the bandstand, and they play "The Stars and Stripes Forever," which, as Sousa predicted, becomes his greatest hit and inspires generations of Marines.