Twilight cast a spell and got herself turned into a filly. Luna has tricked Celestia into believing there is no cure, and now as Luna tries to 'find a cure' Celestia has to look after Twilight the Alicorn; filly version. And heaven knows Alicorns are not born often for a reason.
Twilight casts a spell that turns her into a filly, and apparently there's no cure and Celesia has to now look after her.
Celestia encounters problems when Queen Chrysalis tries to take over Canterlot again, and she is forced to use her failsafe on all ponies in the room. Well, two Alicorns, a Draconequus and a changeling.