In 2999, all people of the world had forgotten the existence of God. A seventeen year old girl in high school, Kisaki Aragaki, has been close to the Church's Sister as a mother. One day Kisaki saw the Church being dismantled by workers and she encountered an Oni (Japanese demon) out of a worker's body. Her father, Kenzo confessed that Kisaki is a reincarnation of her ancestress who fought against Oni with a Japanese saber called Katana of Oni. Then Kisaki took the inheritance of Katana of Oni and faced down the Oni.