The main character in the book is 10-year-old Bud Caldwell, an orphan who has run away from an abusive foster home and is searching for his father. Other important characters are Herman E. Calloway, the senior member in a jazz band who turns out to be Bud's grandfather, and Miss Thomas, the band's singer, a sensitive and understanding woman who facilitates Calloway's and Bud's bonding. Less developed characters include Bud's best friend Bugs, Deza Malone, who gives Bud his first kiss, the Amos, mission, and Hooverville families, and band members Mr. Jimmy and Steady Eddie, who give Bud his own horn.
The story is resolved when Calloway acknowledges Bud as his grandson. The ending is cautiously hopeful, with Bud finding a home with the band and embarking upon a relationship with his grandfather.