15 years ago, the world discovered magic as well as mythical beings from another dimension. Humankind learned to summon these dangerous mythical beings and used their power to great effect. Seven countries formed contracts with these "gods" and gained their allegiance as a result. Summoners on their 14th birthdays will be chosen by a God and given a stigma. Schools have been set up to train summoners and magical swordsmen as a result.
15 years later after the finding of magic, Kazuki Hayashizaki is chosen as a summoner and given a stigma to indicate that. It is surprising because males aren't chosen as summoners due to their low amount of mana compared to females. As the first male to become a summoner, Kazuki is under a lot of scrutiny by almost everyone.
To complicate matters, his sister who is a magical swordsman is against him going to summoner school to train there as a summoner. Because summoners and magical swordsman are in conflict where summoners think of magical swordsmen as inferior, his sister naturally is against him going there. She'll stop him even if it means going to war.
Kazuki, however, is determined to thrive in summoner school and is already determined to give it his all. Even if he's the lowest ranked student at Rank E and his fellow students intend to demean him, he'll prove himself as the strongest summoner.