"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."
In the "Great Disaster," Japan has been thrown into chaos by onmyouji (a kind of magician). Harutora Tsuchimikado was born into an onmyouji family, but has no power whatsoever. His childhood friend Natsume (another member of the Tsuchimikado family) appears to him one day to take him up the promise he made long ago to become her shikigami (familiar). A battle between onmyouji is about to begin.