Princess Celestia, Princess Luna, and Princess Twilight arrive back in Equestria after a 4- day battle with Queen Chrysalis and her Changelings in Saddle Arabia. Due to the restless fighting, their alicorn powers have weaken. As the Summer Sun Celebration arrives, word spreads that an uncontrollable disaster may obliterate Equestria and all its inhabitants.
All hope seemed lost…if it wasn’t for One.
This is in between the Seasons 4 and 5.