After their annual free concert at Chicago's Dearborn Settlement, Benny Goodman and his band are packing up to move on to their next engagement at a military camp, when a kid, Tony Birch, steals Goodman's clarinet. Goodman and Popsie pursue him to a tenement flat where he has led them to hear his brother, Johnny Birch, play the trombone. Goodman offers him a job, over Popsie's protest, with the band. Aboard the train, Johnny accidentally enters the compartment of the band's singer, Pat Sterling, and gets his face slapped. At the military camp, which turns out to be a boy's military academy (which accounts for juvenile players such as Dickie Moore and Harry McKin running around with such titles as General and Major), Goodman finds a real audience in the jive-mad, jitterbug kids. Masquerading as a sweet, 16-year-old girl friend of one of the cadets who is, in reality, her nephew, Trudy Wilson meets her old friend Goodman who introduces her to Johnny, whose music she admired. However, Johnny brushes her off, thinking she is just a kid. Months later, while practicing for their opening at the exclusive Leopard Room in NYC, Johnny runs into Trudy, and is furious that she has followed him to New York. Pat tells Johnny that Trudy is the débutante daughter of Norman Wilson, who runs the symphony association. Johnny decides she is out of his league but they fall for each other when he invites her to a jam session. At a swank party at Trudy's house, where Goodman plays the Mozart Quintet, one of the snobbish guests makes a remark when Johnny's brother Tony, sister Helen and his mother suddenly appear. A brawl ensues and Johnny, feeling he has disgraced himself in front of Trudy, flings his resignation at Goodman and stalks out. Pat's manager gets her to break with Goodman, sets up Johnny as a name band, and hires the Goodman band to join.