Shuuji and Chise are third‑year students at a high‑school in Hokkaido, and their relationship develops slowly as the shy Chise finally confesses her feelings to Shuuji. They begin exchanging diary entries, each entry awkward yet revealing their growing connection. One day, while Shuuji tries to escape a sudden air raid over Sapporo, he witnesses a scene that stays with him forever: Chise, wielding a massive weapon that seems to be part of her arm, takes down enemy fighters one by one. This moment reveals that Chise is the ultimate weapon, whose destructive power is essential to the war.