Everypony is preparing for the Summer Sun Celebration, until Applejack gets a letter from Princess Twilight stating that Canterlot is under attack once again, and that she needs their help. About this time, Applejack notices that Applebloom is gone!

While all this is happening, a Princess from the Northern Kingdom of Frostmare hears of the chaos in Canterlot, and decides to go assist.

With the Cutie Mark Crusaders leaderless, as well as a new threat threatening all of Equestria, will the main six, with the help of their new friend, be able to defeat the strange new enemy? Will Canterlot, Ponyville, and the Eevrfree Forest become a warzone?
And most of all, will each pony be able to retain their special talents, even without their horns and wings?