On a trip to Manehattan to attend Equestria's most famous fashion show, Applejack and Rarity help each other discover important truths about who they really are. Also, there is dancing.
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.