The evil spider summons his bat to attack the fly circus. When the populace flees, it's up to the performers to save the day.
This is a pretty good Terrytoon for the era. The bats are particularly creepy. In fact, this looks like it was intended for a Halloween cartoon and would have been very effective for that season.... except it was originally released in June. Was this canny planning on the distributors' part, to have it in repertory by the time October rolled around, or was it a matter of just pushing product out the door before they went bust and sold out to Fox? It's hard to say from this distance. Fox was in the process of being swallowed by Zanuck's 20th Century Productions and who, if anyone, would be left standing when the music stopped was a shadowy question.