Princess Celestia couldn't do it. She could not bare the idea of sending her sister to the moon. Even if it was some long, lanky, and monstrous version of her. Instead, they made a compromise. Instead of taking over Equastria and have a hooffull of protesting ponies, Celestia gave her a chunk of land too rule, where it would forever be night.

A thousand years later, a rainboom in the now hidden town of Ponyville, and a very inquisitive unicorn..she realizes they're may be way to get her sister back, and let the ponies of Ponyville live in the normalcy and sun.