After starting the day by raising the Sun, Celestia was sure that nothing could possibly ruin her day. Unfortunately for her, something had happened to her sister. She really thought Luna would have learned after the last time.
Princess Celestia's been dealing with some rather embrassing issues for so long, and she decides it's finally time to do something about it. Fortunately, she just so happens to have a sister and a student more than willing to lend a helping hoof.
Cadance didn't know what to expect when her formerly banished Aunt Luna returned from her thousand year imprisonment in the moon. But, she's quite entertained by the hilarity.
The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.