After Cheerilee gets tackled by Starlight Glimmer into her closet, which won't open, they end up being stuck for 2 hours. Who knows what will happen in that time.
Sunset Shimmer is now reformed, but everyone still avoids her, except Twilight's friends. She tries really hard to show that she has changed and hadn't meant what she had done. The new girl at Canterlot High doesn't make it much easier.
When Twilight meets an alicorn princess named Autumn Breeze, she finds out that not only are her and her friends are in danger, but also all of Equestria.