The portal to a world where all ponies were born as humans is not the only portal Celestia guards. There are many portals, and all must be guarded with her life, as harmony hangs in the balance if even one were to break a barrier between worlds.
But one has broken, and Celestia and Luna have been plunged into another world that is shrouded in an everlasting winter. The God of Autumn and Winter and the Goddess of Spring and Summer have both fallen into a deep sleep, and without their presence the world of Amaravir hangs precariously on the edge of destruction. Can the princesses find the gods and awaken them before the evil that put them to their rest consumes Amaravir? And can they fix the shattered portal, lest Equestria fall under the same peril?