Tanya Degurechaff is a young soldier infamous for predatorial-like ruthlessness and an uncanny tactical aptitude, earning her the nickname "Devil of the Rhine." However, beneath her innocuous appearance lies the soul of a man who challenged Being X, the self-proclaimed God, to a battle of wits—which resulted in him being reincarnated as a little girl into a world of magical warfare. Tanya, hellbent on resistance, intends to rise through the ranks of her country's military as it progressively descends into world war, with only Being X proving to be the most formidable hurdle to resuming the quiet life she once knew. However, her astute acts and fighting initiative have an unanticipated consequence: driving the vast Empire to become one of the world's most powerful countries.