Every pony deserves a happy ending; every pony deserves to be equal. At the end of time, Starlight Glimmer wanders the desert in search of a way to give them both... but if forced to choose only one, she already knows what she'll do.
A dark, reality-warping Malevolence causes Rarity to be stuck in a pocket of temporal feedback. With but one week to work with, Rarity must stop the Malevolence's impending return, or be forced to do it again. And again. And again.