Around the the 37th year of the Meiji Era (1904) in the midst of the Russo-Japanese war, the small Japanese army, in need of assistance, uses its special flying (thanks to a benevolent demon) ship, the Amanohara, to attack Russia's major base at Port Arthur (Lushun).

Umakai Shintarou, a Japanese diplomat, returns from Russia and goes to Sapporo to teach at a girls academy. However, that girls academy is not typical - it is on board the Amanohara, and the five girls Shintarou teaches are known as the Lime Unit - girls with the ability to summon powerful beings to fight for them.

[Edited from AnimeNewsNetwork]