In 2013 of modern-era Tokyo, you’re Japanese salaryman, in the moment of being murdered by a disgruntled subordinate whom you had fired due to poor performance at work, is confronted by an entity that declares itself to be God who condemns you for not having 'faith'. Your disbelieves in its existence, criticises its various statements from your perspective as an atheist and mockingly terms it as 'Being X'. The entity decides to reincarnate you into a world where you would face sufficiently difficult circumstances to turn to Being X for help.

You’re reborn as Tanya Degurechaff, an orphaned girl in an alternate universe's equivalent of Imperial Germany, known as the Empire, in which World War I has been delayed until the 1920s and where magic has been incorporated into the military. According to Being X, if you either does not die a natural death or refuses to have faith in it, your soul will leave the cycle of reincarnation and will be sent to hell for the countless sins that you has committed in your previous life. In search for an escape, you decides to join the Empire's Mage Corps and fight in the 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 you’re now forced to speak with a young girl's lips, you soon turns into a ruthless soldier who prioritizes efficiency and your own career over anything else, even the lives of those beneath you, especially those that get on your bad side.