Set in the BoD Universe. When Twinkle abruptly renters Zany, Applespike, and Jewel’s lives, they learn of a power Twinkle has been keeping hidden for 15 years. Now, they must help her overcome a problem that may keep one of them stuck in a nightmare.

Princess Celestia doesn't even know when the middle of her life will be. She's just assuming it's happened right now. And she's freaking out.