A rich kid makes fun of Mickey because he's poor. Mickey is angry and announces that he's going to beat the rich kid in some horse race (wow...that's responsible parenting...letting kids race horses!). The problem is that Mickey doesn't have a horse. Out of desperation, he borrows a mule and tries to make the best of it.
This is Mickey Rooney's final film in his Mickey McGuire series--and after seeing it, I see he made a great choice moving on to other projects! The film is essentially a knockoff of an Our Gang comedy--except that the Our Gang series was better written, better directed and better acted. I guess that doesn't leave you with much...
By the way, note that Mickey's brother in the series was Billy Barty--the famous midget. Although he was about 9, he played a kid about 4 years-old. That must have been a bit depressing for Barty. Of course, being in this film sure wouldn't have made him feel so hot, either.