Twilight noticed strange things happening in Ponyville, but she wasn't alone. Luna found out about this mystery too and sent out moon princesses to search for a certain pony. That certain pony is an average unicorn filly, who finds out about this legend in a book, and begins having dreams on meating those princesses. At least she thinks they are dreams that is.
Things later get harder and more mysteriouse as the sun dies, Sunrise discovers Twilight, who 4,000 years ago was wrote in the legend Sunrise read. They then discover, not a very big thing, and even a lesson they both had completely forgot about.