After an argument breaks out, Celestia tells her sister, Luna, as a prank that the moon is now made out of cheese. Of course, Luna goes to see for herself. What she doesn't realise that getting to the moon isn't really that easy.
Twilight is the Princess of the Stars, but there's something about them that Celestia neglected to mention. When one falls into her library, she realizes that her job is a lot harder than she thought.