It was Meiji 37 or year 1904 (somewhere in the timeline of Sakura Taisen), when The Great Empire of Japan started to increase its national power quickly by the westernization during the Meiji era. On the other part of the world, Czar Russia was going to extend its hegemony to the East and hence started a war against Japan. The war is known as Russo-Japanese war right now. Japanese army with quick series of victories over Russia astounded the world. The war culminated with the fall of Russia’s Port Arthur (Lushun in China) to the hand of Japanese army.
The story of this anime (based on a h-game by Elf) starts from when the Japanese army is getting readied to attack Port Arthur by establishing a training camp (called Tendai Genjo Gakuin?) for female pilots in Hokkaido. Umakai Shintaro, a former diplomat who was residing in Russia once, is chosen to supervise the training.
Although five girls (Sanada Momen, Honda Sarasa, Katou Asa, Fukushima Kinu, Kuroda Rinzu and Ichijouji Sumi) who are assigned to him overwhelm him, his sense as a teacher brings him to guide these girls to complete their secret mission, although he does not even know what the secret mission is (which is to destroy the mysterious Russian Army at Port Arthur in Lushun).