When Twilight's portals release Tirek into the human world and the humans get turned into ponies, they have to unite with their ponyselves to defeat him and save both world's.
While everyone was gone, including Human Twilight, Princess Twilight spends her Summer vacation in the Mirror World by meeting people she missed when she was away.
Suri Polomare needs a decent language arts paper to stay in the Crystal Prep fashion program. Coco Pommel has a decent language arts paper. And she doesn't suspect a thing...