Tanya Degurechaff is a soldier who has the appearance of an innocent young girl. But behind her cute facade lies the soul of a ruthless salaryman who challenged "Being X"—the self-proclaimed God who reincarnated the man into an industrial world of magic. By doing this, Being X hopes to prove that, if the situation were dire enough, even a non-believer like this man would pray for God's help.

Opposing Being X's desires to make her suffer, Tanya enlists in the military with the hope of living a peaceful life. She manages to impress the higher-ups with her aptitude for war, swiftly rising through the ranks. However, her country soon participates in the ongoing world war, quickly crushing her dream. In an attempt to return to her calm she starts using any merciless means to obliterate her opponents. Thus begins the saga of Tanya the Evil and the Empire as they seek world domination.