Stan Kenton and his orchestra show up in this soundie playing pretty hot swing amidst a lot of editing to show their rapid rise.
Soundies were musical videos. They were played on video jukeboxes called the Mills Panoram, and from about 1940 through 1947, more than 2500 soundies were produced. They were valuable training grounds for up-and-coming talent. Doris Day's first movie was a soundie; so was Nat King Cole's. For a dime, you got the performers doing their songs. Then production stopped, the machines went away and people forgot about them.
This one shows a lot more editing than any other soundie I have seen.