As punishment for her crimes, the Elements have cursed Sunset Shimmer to do favors for anyone who asks. Lucky her. And then there's that other Twilight Sparkle she keeps running into...
When one fateful search through the Everfree Forest leads Rarity to a secret library inhabited by the spirit of an ancient alicorn princess, she realises that it may be time to start believing in fairy tales.
When Twilight overhears a private conversation, she has to come to terms with how she feels about herself, love, and a certain somepony. All while weighing her public image against her personal desires.
For Twilight, the increased lifespan that came with princesshood had forced her to outlive her friends, but thanks to Rarity, she will learn that centuries could be lived in but a single day.