Twilight and Shining Armor are hired to help with a huge group project that involves drawing down an ice asteroid to Appleloosa to fill their reservoirs. But Queen Chrysalis, in another gamble to take over Equestria, intends to sabotage the attempt by bringing down the asteroid upon Celestia, Luna, and all of Appleloosa. However, Twilight and her brother get help from an unlikely ally: a changeling named Mirror who has unique shape-shifting abilities and whose loyalties are torn between her queen and her conscience.