This isn't really all that spoilery, so I'm not putting it behind a break. "When Rainbow Dash discovers that the new A.K. Yearling book she has been eagerly anticipating is delayed, she recruits her reluctant pony friends to spring into action to ensure the book gets written. However, when the ponies arrive to help the famous author, they soon discover that A.K. Yearling has a secret that sends Rainbow Dash and her pony friends on an unexpected mission."