The CMC is bored out of their minds with nothing to do on a lazy summer afternoon, so they decide to drop in on a friend to see if there's anything fun to do there.
Sweetie Derelle, the alternate reality Sweetie Belle, has arrived in Equestria to save everyone. That includes driving a bus at high speed through Canterlot to make sure it doesn't explode. Also into Canterlot Castle. Specifically Celestia's Bedroom.