Twilight is having a sleepover with her best friend (and crush) Sunset Shimmer, and they have the house all to themselves. Unbeknownst to Twilight, Sunset has some plans for the evening.
After a portal malfunction threatens to seal the human world and Equestria apart, the Rainbooms decide to write letters of friendship and appreciation to Princess Twilight.
Princess Twilight tells the tale of her many adventures to her latest student and how she remembers her friends from long ago. But she can't tarry too long, for she has a long-overdue meeting to attend.
When the sun stays in the sky for three days straight, everybody turns to the princesses for guidance. Unfortunately, Luna is nowhere to be found, and Celestia seems to have something else on her mind.
After a mishap with yet another teleportation spell, Trixie and Starlight Glimmer find themselves on an island in the middle of the ocean with no magical energy left. Starlight is sure this is Trixie's fault. Trixie is sure Star needs to chill out.
One fateful morning, Princess Luna finds Celestia has turned into a megalomaniacal tyrant with one clear goal in mind: to be sent to the moon. She even has her bags packed and everything.
Luna reread the graffiti, but it came no closer to making sense. “Princest Is Wincest,” it said. It was confusing enough to be offensive; did nopony spell correctly anymore? Perhaps Tia would be able to explain it.
The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.