"Hold on... What kind of spell again?"
"A Princess of the Day spell. It swaps my alicorn nature onto another pony, making them a Princess in my place for a day."

Twilight is a little fed up as the newly-minted Princess of Friendship Magic. So she uses a spell to give each of her friends a chance to see what it's like being the alicorn Princess of Ponyville in her place. And learns way more than she bargained for in the process.