In the near future, Earth is under invasion by otherworldly monsters known as giju, or false beasts. Humanity is locked in an endless defensive war. Conventional weapons are almost completely ineffective against these creatures. The only effective countermeasure is the Valkyrie Dress, a powered suit of battle armor equipped with mysterious abilities that can only be operated by women. To train and manage the female pilots who wear these steel dresses, the Third Academy is established as a dedicated institution.

Into this setting arrives Ren Hifumi, the world's sole test pilot for a prototype anti-giju humanoid weapon. He is the product of a risky surgery with a success rate of less than ten percent, and his experimental weapon has never had a successful trial run. Now, as a man transferred to an academy of young women who fly through the sky in powered armor, he must navigate a school environment while preparing to face the monsters that threaten humanity.

The story follows the tagline that when a soldier sets foot on a maiden's battlefield, a story without heroes begins. Ren is a hardened mercenary who has lost his comrades, carrying a history of war and a desire for vengeance. He is scouted by Hayase Maya, the ace pilot of the Third Academy, and assigned from the reserve unit, Class 5, to the combat unit, Class 3, known as Boots Cat. However, the atmosphere inside Class 3 is tense and fraught with discord. Hayase Maya, who should be the star of the unit, is isolated and alienated by her peers, a situation so strained that even the socially inept Ren becomes concerned for her well-being.

Amid this simmering tension, Ren is tasked with a mock battle against a student from Class 2 named Seiryu Seion. She is a genius swordswoman who wields the ancient martial art of the Solitary Moon Azure Sky Style. This confrontation serves as a key early challenge that tests Ren's abilities and begins to reshape the dynamics within the academy. The narrative explores themes of vengeance, camaraderie, and the struggle to find purpose on a battlefield where traditional heroes are absent.