When Discord learns about Nightmare Night, he decides he absolutely cannot pass up a chance to sow a little chaos. After all, he promised to be good, but only most of the time.

With all of Ponyville, including the bearers of the Elements of Harmony themselves, caught in Discord's little sideshow, only three ponies are in a position to do anything about it.

The fate of Ponyville and Nightmare Night itself are in the hooves of Apple Bloom, Sweetie Belle, and Scootaloo. The Cutie Mark Crusaders.