"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."