Mayor Mare has a bathroom on the other side of Town hall she likes to visit for the privacy. However, one day she finds a door that previously didn't exist in the hall leading to. It is locked, and so she makes efforts to find out where it leads and how to open it.
Unfortunately for her, it goes places that aren't anywhere near Ponyville, and are dangerous to her home. As mayor, she has a duty to make sure that it remains closed, and the visitors from the other side can't get through.