Tanya Degurechaff is a young soldier known for predatorial-like ruthlessness and an uncanny tactical aptitude, earning her the moniker "Devil of the Rhine." However, beneath her innocent exterior lies the soul of a man who challenged Being X, the self-proclaimed God, to a battle of wits—resulting in him being reincarnated as a little girl into a world of magical warfare. Tanya, hell-bent on resistance, vows to rise through the ranks of her country's military as it descends into world war, with only Being X standing in her way of resuming the calm life she once knew. However, her foresight and military prowess have an unanticipated consequence: driving the formidable Empire into one of the world's most powerful countries.