A sudden and savage changeling invasion is threatening ponykind, forcing Celestia to issue the Fortress Equestria Decree: the country's border must be hold at all cost. Now, ponies and changelings are locked in a bloody conflict, with no end in sight.
Fortunately, the Princess faithful student might know how to turn the tide of war. Unfortunately, the Princess has sent her away from her laboratory in Canterlot, and worst of all: she's going to the frontlines of Ponyville.