Margie and the gang decide to "put on a show".
There is a definite nostalgic aspect to this episode, the kids quickly (way TOO quickly) learn to master their instruments and not only play the school dance but go on to greater things by performing in a speakeasy. Aside from highlighting the usual power of teen ingenuity, Margie's father Harvey is his normally eccentric self, playing with a crystal set radio and later, getting punched in the eye when defending his daughter's honor. In an early scene, there is just something charming about Margie excusing her father from her "official" meeting of the High School committee for planning the Spring dance. Another steadfast element is Margie's mother, conservative in her hopes for her daughter. Its hard to actually separate a sense of 60s sitcom from the 1920s setting, maybe a reaason why the series wasn't unique enough to last.
All-in-all, there is quite bit of sentiment in the performances in this episode, a lot of an eager sense of pulling together on the part of the kids - and at the same time - the input of adults who really "know the score". I would actually watch some other installments of this series if they were airing today.