Six months after Twilight and her friends defeated Nightmare Moon Applejack and Rarity receive letters they would have never expected or wanted:

I've taken Apple Bloom, Sweetie Belle and Scootaloo if you ever wish to see them again bring the Elements of Harmony and their bearers to the site of your victory over Nightmare Moon this evening.

Do not inform anyone but the bearers of this, or the Cutie Mark Crusaders as these foals call themselves will never be seen again. Just like the old fool you charged with their safety