Many years after escaping from Ponyville, Pinkie Pie's evil clone senses a disaster in Ponyville affecting everycreature, including her. So she takes a break from her villainous routine, to stop it by any means necessary.
What's it like, knowing one of your best friends tried to kill almost everypony in the world who looked like you? Sometimes Pinkie Pie would wake up from a nightmare, and remember.
Twilight Sparkle didn't consider what might happen when she used an "Object Possession" spell to check on her student. Now she has to live with the consequences. Forever.