A man known as the strongest mercenary, Ainz. He had a daughter-like strongest mercenary girl named Sierra whom he had picked up and raised. ...However──, "I will retire from being a mercenary." Ainz's sudden retirement declaration. Sierra, who knows nothing of the world outside the battlefield, ends up attending the royal academy alone at Ainz's wish. It was constricting to live while hiding her mercenary background, but Sierra meets a noble girl named Alna and develops a friendship. However, the situation changes drastically when Alna gets caught up in the royal succession issue and an assassin's hand approaches her. To protect Alna, Sierra resolves to use the strongest power she has cultivated as a mercenary. A naive yet powerful mercenary girl unfolds a girls' academy battle fantasy! Here it begins!!