Takeru Ohyama, a young normal yet perverted minded guy, got accepted and now goes to a school that, unknown to him, was where combat and magic is used. On the first day, he meets again after three years: Haruko Amaya, his childhood friend, Inaho Kushiya, a girl who says she's his fiance, and Kodama Himegami, a blonde who wants to kill him. Finding out that there are many girls there that just don't like him, he is told that everyone there uses a special magic ability while using an item or weapon called a Maken. Later, he finds that no Maken is acceptable for him and doesn't know what to do, since in this school the students get into duels that showcase their magic and combat power, of which he seemingly has none.
"The fourth true ancestor" is a legendarily strong vampire that shouldn't exist. Accompanied by the 12 "gun-beasts," this phantom vampire is said to have appeared in Japan. In order to find and kill this "fourth true ancestor," the government and the Lion King Organization order the dispatch of attack mages called "sword-maidens." For some reason an apprentice named Yukina Himeragi was chosen for the task. Wielding the strongest spiritual spear, Yukina comes to the special demon district Itokami City. There she encounters Kojou Akatsuki and learns the true nature of the "fourth true ancestor."