Warning: Spoilers follow
Shuji is main character,who is high school student,lives in environs of Hokkaido. Shuji was declared by Chise that She likes Shuji. But Chise is very shy and Shuji has a bitter tongue. So they started correspond diary to communicate as a couple.
One day, when Shuji went shopping in Sapporo,unknown bombers strucked the city at broad daylight. He run from bombs. Then,a rapid and small flying-object shot down enemy bombers. And,he witnessed it is Chise who has wing and weapons. He noticed that Chise was converted into the ultimate weapon to fight against opponent arimies.
The manga has a very hot war. It represents battles and reposers at front. But the dipromatic particulars of the war is not given to readers.
For example, we can't find out ,why the war broke out, what is the war all about, and what country(s) is the Japanese Self-Defense Forces and Chise fight against. Moreover, why Chise was chosen the one converted into the ultimate weapon, is not described.