When clarinetist Benny Goodman and his band play their annual free concert at Chicago's Dearborn Settlement House, where Benny grew up, feisty youngster Tony Birch asks the famous musician to come to his house to listen to his older brother Johnny play trombone. When Benny demurs, Tony grabs his clarinet and dashes through the streets until he arrives at the Birch family's tenement apartment. Benny and his manager, Popsy, and pianist, Jess Stacy, are in hot pursuit, but when they hear Johnny play, their anger is forgotten and Benny and Jess jam with the talented young man. Benny offers Johnny a job, which the suspicious Johnny at first rejects, but his mother convinces him to accept. Popsy, who had wanted to become the band's next trombonist, is upset, but puts up with Benny's whim and introduces Johnny to the other band members at the train station. On the train, Johnny bumps into Pat Sterling, who he assumes is an autograph hound until Popsy reveals that she is the band's vocalist. After Johnny apologizes, Benny warns the flirtatious, sophisticated Pat to leave the naïve Johnny alone. The next day, the band prepares to perform at a military ball and are stunned when their host "General Carmichael" greets them and turns out to be a teenager from the local military academy. Benny gracefully sticks to his commitment, however, and Carmichael brings his adult aunt, Trudy Wilson, who calls him by his nickname, "Mogie," convinces Trudy to dress younger so that the other fellows will not know he does not have a real date. When she appears in her adolescent garb, even Benny, who knows Trudy's family, is surprised. Despite his nervousness, Johnny performs well, especially in the solos that Benny generously allows him. Benny confides in Trudy that he is worried that Johnny has the "wrong kind of confidence" and will easily be misled by Pat. Benny introduces Trudy to Johnny, hoping that she will win him away from Pat, but Johnny assumes that she is a teenager and Trudy laughingly perpetuates the charade. Later, Benny asks Trudy to visit the band in New York, but when she arrives, the jealous Pat tries to keep Johnny away from her. When he talks with Trudy, however, Johnny is impressed by her kindness and takes her to a jam session. Benny, knowing that Johnny is not ready, gently tells him not to join in, but Johnny gets angry and storms out. Trudy chastises Johnny for his temper, but the arrogant young man asserts that he is determined to become famous so that he can rise to her social level. As three months pass, Johnny's fame and ego grow, until one day, Pat takes him to meet her slick agent, Lester Barnes. Hoping to make a quick buck, Les asks Johnny to leave Benny and start his own band with Pat as his vocalist. Johnny agrees, although his family and Popsy criticize him for leaving Benny. Johnny steals Benny's musicians as well, and although business is good at first, Johnny does not have the appeal that Benny does and the crowds desert them. The men and Pat return to Benny, and although Johnny realizes the mistake he has made, he returns to Chicago and resumes his factory work. Later, Benny and the band play at the Settlement House, and Popsy goes to visit Johnny. The humbled musician admits that he should have treated Benny and Trudy better but still refuses to return to the band. Popsy finally carries Johnny to Benny's concert, and there, Johnny reconciles with both Benny and Trudy.
