As a child, junior high student Nanoka Kiba lost her parents in an accident at a shopping arcade. Passing through a portal in that same arcade, she lands in the Taisho era, where the exorcist Mao reluctantly rescues her from the jaws of a grotesque yokai. Back in the present, she discovers she has gained strange new abilities, so she returns to the past for answers and gets caught up in Mao's investigation of a string of gruesome murders. There she learns that Mao, an onmyoji who has lived for nine hundred years, is cursed by a cat demon named Byoki, and so is his sword.