Needing advice on an important project, Twilight, Tempest, and their friends work to reopen the mirror portal, and rescue a former student of Celestia's... Sunset Shimmer.

Note: This story assumes the backstory for Sunset presented in: Sunset -- Part I and Part II.

For more of this AU, see: By the Light of the Moon

And also check out the series, beginning with Not Exactly Friends.