The past always comes back to haunt the present, there is no escaping the inevitable. For decades, Celestia has kept a secret from her student. A secret she made out of pain, and through pain will be revealed for all to see; for Twilight Sparkle is
Foals are often more than a hoof full, particularly if your experience with them is limited. When you make a rash decision, deciding to take care of a filly with near unlimited power for example; it becomes even more complicated.
A much older, arguably wiser, somewhat darker and definitely crazier Twilight Sparkle switches minds with her younger self to FIX EVERYTHING that went wrong before