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.
As punishment for her crimes, the Elements have cursed Sunset Shimmer to do favors for anyone who asks. Lucky her. And then there's that other Twilight Sparkle she keeps running into...