An atheist Japanese salaryman, in the moment of being murdered, is confronted by a self-righteous entity he refers to as "Being X" who condemns the man for not believing that "Being X" is God and sentences him to be reborn in a world that is similar to WWI Europe mixed with early aspects of World War II and magic, in an empire torn apart by countless wars with all nearby countries. The salaryman is reborn as a girl, Tanya Degurechaff. According to "Being X", if she's not able to die a natural death, or she refuses to have faith in Him, her soul will leave the circle and be sent to hell for the countless sins that Tanya has committed in her previous life. In search for an escape, Tanya decides to enter into the Empire's Mages Division and fight in the world war, hoping to reach a high enough rank as fast as possible to remain far from the battlefield and in this way avoid the risk of being killed. Even if she's now forced to speak with a young girl's lisp, Tanya soon turns into a ruthless soldier who prioritizes efficiency and her own career over anything else, even the lives of those beneath her, especially those that get on her bad side.