In a near future where humanity has learned to call mythical beings named Diva out of a parallel dimension, Kazuki Hayashizaki becomes the first male ever chosen as a summoner. Summoning has been a women's art until now, and his place in an all-female summoner class is opposed from every direction, most fiercely by his own sister Kanae, the school's best magic swordsman. Kazuki sets out to prove himself while pulled between the sword and the summoning he has just been handed.