Japan is in a state of prolonged civil war, and high school students are mobilized into armed volunteer units to maintain order. In the semi-independent neutral city of N-shi, Seishin Girls' Academy has made these volunteer activities a compulsory part of its curriculum. Students are tasked with patrolling their district, apprehending criminals, and even authorized to use lethal force when necessary.

The story follows Shimon Hanazawa, a diligent and compassionate advanced‑placement student who transfers into the academy and chooses to serve as a trooper. She is paired with Momoko Takasaka, a volatile, heavily tattooed classmate who also acts as her instructor. Momoko is abrasive and quick to criticize Shimon’s reluctance to hurt opponents, but she grudgingly respects the newcomer’s tactical mind and combat skill. Despite their constant friction, the two gradually learn to trust each other and coordinate effectively in the field.

Shimon’s idealistic approach—prioritizing the safety of civilians and attempting to subdue foes without killing—often puts her at a disadvantage against hardened criminals and armed factions operating within the city. Momoko, by contrast, is ruthless and pragmatic, shaped by the endless conflict. Their dynamic is further complicated by internal school politics, rival military academies, and an influx of war refugees that strains the neutral zone’s fragile stability. When a mutual acquaintance is murdered, the pair take on an investigation that forces them to confront the darkest edges of their world, blurring the line between justice and survival.