Sunset arrives in the human world as a pony in the middle of a busy freeway in Britain and physics happen. Circumstances conspire for her to end up going to Hogwarts in Harry's year as Luna Lovegood's pet.
Twilight Sparkle is a bit underwhelmed with her ascension to alicornhood and, after a disastrous coronation ceremony, it becomes clear that something is missing. It'd be a shame if somepony else got to it before she did.
Changeling Twilight Sparkle and her number-one assistant, Sunset Shimmer, try to study magic without learning any wholesome lessons of friendship. They fail.