Thanks to Twilight Sparkle's meddling, a critical element of Starlight Glimmer's stolen time-travel spell has failed, casting her across the timelines and stranding her in one where she doesn't belong.
But Nightmare Moon, the new ruler of Equestria, isn't so bad. Ponies still have lives and a few important freedoms, and that has convinced Starlight Glimmer she was right. Twilight and her friends aren't that special! With moral certitude as her guide, Starlight once more finds her place as a preacher of her anti-cutie mark ideology.
Then Nightmare Moon notices.
This story was inspired entirely by a conversation with Penalt, Damaged, and HiddenMaster.