A young trumpeter rises through the jazz world and finds love.

In New Orleans in 1906, the Congo Square Building, formerly the site of slave auctions, now serves as an African-American employment bureau. Nearby, in an African- American college of music, an instructor is teaching his pupils to play Bach. Seven-year-old Reggie Tearbone, who is learning to play the cornet, is unable to follow the sheet music, however, and after playing a few bars, begins to improvise a jazz composition. Reggie lives with his mother Ella, who is employed as a servant in the home of architect George Latimer, a member of the once aristocratic but now impoverished Latimer family. One day, Latimer's old friend, Steve Porter, comes to visit from Chicago, accompanied by his son Paul. Upon learning of Latimer's financial problems, Porter persuades the architect and his young daughter Kit to go back home to Chicago with him. Ella accompanies the family, but Reggie, who has secured a job playing in King Jeffers' Basin Street Band, remains in New Orleans. As the family travels up the Mississippi River, they hear the music of Memphis and St. Louis. By 1916, a new style of jazz has developed out of Ragtime. On Kit's seventeenth birthday, Latimer and the Porters leave her to celebrate alone while they entertain some clients. Lonely, Kit wanders out onto the street and there meets Johnny Schumacher, a struggling young cornetist. Johnny takes Kit to a party at the apartment of musical promoter Smiley Jackson, and when Kit incites a riot with her New Orleans-style piano playing, she is arrested. At her trial, Kit is acquitted when she wins over the jury with a rousing rendition of boogie-woogie piano. The advent of World War I transforms both American music and the Latimer family. When the war forces the closure of Basin Street, Reggie, now known as "Rex Tearbone, King of the Cornet," travels to Chicago with Jackson, now a successful music impresario. Paul, now engaged to Kit, bids her farewell as he goes off to war. After Paul is killed in combat, Johnny and Kit realize that they love each other. Soon after the war ends, they are married, and Johnny gets a job on the road, playing in a large jazz orchestra. Kit, protesting that he will never be happy playing the circumscribed repetoire of the orchestra, refuses to travel with him. Becoming disillusioned when he is denied his promised featured spot in the band, Johnny quits and, after wandering around finds new musical inspiration in the hobo "jungles." He receives an offer from Smiley, who is now a successful booker for whom Kit is working, to come to New York. There he reconciles with Kit, while Smiley arranges some bookings for his new band. At first, Johnny's new sound is a failure, then his audience realizes its dance potential. Dubbed "swing," Johnny's music revolutionizes the sound of jazz, and all ends happily for Kit and Johnny as they listen to a jam session featuring several great jazz musicians.