Skylar has a mirror in her closet that's actually a portal to Equestria. When she goes back for the first time in a month she finds Sunset Shimmer will be hanging around with her and the 6. Sunset has a nightmare and somehow the 8 get stuck in it.

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.