A birthday present for EpicLuna

Twilight Sparkle is an intelligent pony, normally well-composed, and blissfully unaware of the state of her plot. But when she overhears a few words not meant for her ears, how will she handle the revelations and the endlessly complex task of determining the condition of her derrière?

Many thanks to Aeteros, DethByGaming, and "Twi" for their help with various parts of the story.

WARNING: Spoilers in the comments