Sunset Shimmer, the most talented student of Princess Celestia of all times, is sent on a mission to the frozen north. Almost a thousand years ago, in a war of dark magic, the legendary Crystal Empire has vanished from the face of Equestria. Now, Princess Celestia sends her best student to explore the remains of the long forgotten Empire.
Sunset Shimmer takes on this new mission to prove herself worthy of becoming an alicorn princess herself. However, what she discovers at the remains of the Crystal Empire might change everything.
In the comic "The fall of Sunset Shimmer", there is no indication how much time passed between Sunset Shimmer's first contact with the mirror and her escape into the human world. This story takes place is this period of time and moves on from that.