The time is UC 1926. The Imperial Army's 203rd Air Mage Battalion led by Major Tanya Degurechaff has won the battle to the south against the Republic's stragglers. They expected to be given a vacation after returning victorious, but instead receive special orders from Staff HQ as soon as they get home. They are told that there were signs of a large-scale deployment near the Empire-Federation border.

Faced with the prospect of a new major enemy, the desperate Empire fans the flame of war.

Meanwhile, an international volunteer army spearheaded by the Commonwealth set foot in Federation territory. As they say, the enemy of an enemy is the best friend you could possibly have. They suffer through misfortune purely out of national interest, and among them is a young girl.

She is Warrant Officer Mary Sue, and she takes up arms hoping to bring the Empire, who killed her father, to justice.

[Source: Crunchyroll]