Hal Le Roy in a dual role - he's a swinging saxophonist Mr. Razzenstill on his way to the country of Sulvania. There, he discovers he is a distant cousin to the country's Prince Rudolph. The prince hates Swing music, and declares that, after his coronation, the music will be banned. Swing lovers swap the identical Le Roys - and, Rudolph is thrown in his dungeon. Will he escape?
The film is far too dependent on the films it spoofs ("The Prisoner of Zenda"). Given that, you can't follow the story; for example, why does Razzenstill decide to try and rescue Rudolph? June Allyson is the princess who seems to fall in love with the other man; it's an interesting, but unresolved, subplot. "The Prisoner of Swing" highlight is Le Roy's expert tap dancing performance following the scene with Ms. Allyson.
** The Prisoner of Swing (6/11/38) Roy Mack ~ Hal Le Roy, June Allyson, Eddie Foy Jr.