The story begins hundreds of years after the TV show. Twilight Sparkle has outlived the rest of her friends, serving as Court Sorceress under Luna's reign. After Celestia loses her mind and succumbs to dementia, Twilight sends herself back in time to be with her friends once again. But she quickly learns that no pony is time's master.
Author's Note: Second story, and in very sharp contrast to my first (Lazy Eye for Fluttershy). If you're wanting a story about nice things.. Go read that story.