From Anime News Network:
In the story, the seasons have changed, and it will soon be the second spring. The poor student older brother Tatsuya and the honor student younger sister Miyuki have finished their first year at their magic high school, and are on their spring break. The two go to their villa on the Ogasawara Island archipelago. After only a small moment of peace, a lone young woman named Kokoa appears before them. She has abandoned the Naval base, and she tells Tatsuya her one wish.
The film will feature a new character named Kokoa, who is a deserter from the National Defense Navy's base, and who ends up
in Miyuki's care. Her appearance is that of a very young woman, but her true character is shrouded in mystery.
k, fine...