Aoi, who inherited the power to see "Ayakashi" from her late grandfather, is one day taken to "Tenjin-ya," a long-established inn in Kakurei, the world where Ayakashi live. There, she is told that her grandfather once made a "certain promise" with the head of the demon god. The content of the promise was that "as collateral for his debt, he would give his granddaughter to the head of the demon god as a bride." (Source: Kodansha, translated)