Sunset Shimmer has fled back to Equestria, forced out of place by her double. The local versions of her friends try to make her feel comfortable, but Sunset is upset and scared. Worse still, she's becoming unstable again. She didn't come back alone.
Princess Celestia must seek out a "pony" she unintentionally wronged many years ago, and will have to live knowing that her good intentions had caused the mare untold sorrow.
As kids, Twilight and Sunset were best friends, but a tragic accident cut their friendship short. Years later, Twilight investigates the incidents at Canterlot High and comes face to face with what appears to be a literal ghost from her past.
People rarely consider their actions, even less consider the consequences. What they might push someone too, or how far they will fall. Sunset Shimmer knows this all too well. Pain makes people do terrible things.
The Mane Seven have united their powers before, and over time their friendship has become stronger than ever. But only now, as graduation looms, do they unite for revenge.