Twilight Sparkle starts to learn about gravity-manipulation spells after Princess Celestia sends her a book on the subject. Casting the spell is easy, but cancelling it, not so much, and Twilight soon finds herself without a hoof on the ground when she becomes the accidental target of her own magic.
On any other day, Twilight could deal with this. But with representatives from the prestigious Canterlot School of Magic due that evening to consider a possible annex in Ponyville, Twilight must find a way to be able to attend the event without revealing her foul-up.