In 2013 of modern-era Tokyo, an unnamed atheist Japanese salary man, in the moment of being murdered by a disgruntled subordinate whom he had fired due to poor performance at work, is confronted by an entity that declares itself to be God who condemns the salary man for not having 'faith'. The salary man disbelieves in its existence, criticizes its various statements from his perspective as an atheist and mockingly terms it as 'Being X'. The entity decides to reincarnate the salary man into a world where he would face sufficiently difficult circumstances to turn to Being X for help. In the moment of his sudden death he is sentenced to be reborn in an alternate universe similar to 1910's Europe, in an empire torn apart by countless wars with all nearby countries. The salary man is reborn as Second Lieutenant Tanya Degurechaff of the Imperial Army, a 9-year-old soldier girl. Plus this, according to God's words, 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, 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.