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.
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.