Twilight Sparkle accidentally leaves her class with the wrong backpack. Normally, this wouldn't be a problem, except that the backpack belongs to the one girl she's had the biggest crush on since moving to Canterlot High.
After developing a new portal that can explore other dimensions, Twilight and Sunset are pulled into an adventure, where they meet even more doppelgängers of themselves. Together they have to save the day, and learn about each other in the process.
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.