A salaryman's optimization instinct doesn't survive contact with actual warfare. Tanya gets reincarnated into a child's body and immediately tries to game a war economy like she'd game a promotion track, except now there's a god actively sabotaging her and real soldiers dying when her calculations fail. The show's actual horror isn't the premise, it's watching someone with perfect corporate logic discover that some systems can't be optimized, only endured.

Once one of Japan's elite office workers, a man is reborn as a little girl after crossing a self-proclaimed god known as Being X, and he lands in a world where wars are fought with magic. Now Tanya Degurechaff, blonde, blue-eyed and outwardly innocent, flies into frontline combat and cuts down her enemies without mercy, earning the nickname "Devil of the Rhine." She values efficiency and promotion above everything else, and she sets out to rise through her nation's military as it slides toward world war, hoping to regain the peaceful life she once had. Being X remains her greatest obstacle, and her sharp tactics carry an unexpected consequence: they help push the Empire toward becoming one of the mightiest powers in history. The result is an action fantasy about a ruthless mind hidden behind a child's face.