Apple Bloom, inspired by her comics, suggests that the Cutie Mark Crusaders should become spies, but Scootaloo and Sweetie Belle don't think much of the idea. This is Equestria, for Celestia's sake; a land where happiness is the default setting, and the word "enemy" is so rarely used it needs a good dusting.
But it seems that Apple Bloom's suggestion is timely; when weird events start occurring around Ponyville, it looks like they'll soon be up to their ears in mysteries. And who knows just how far they will have to go to unravel them...