Set in Kashihara, Nara, this story follows a young boy named Akihito Kanbara who is an immortal being and half a spirit.

After Akihito saves a girl named Mirai Kuriyama from committing suicide, he learns that she is a spirit world warrior their lives become intertwined.

One day, high school student Akihito Kanbara instinctively goes to save his fellow schoolmate, Mirai Kuriyama, from committing suicide.

After learning that Mirai is a Spirit World Warrior (異界士, Ikaishi)—specialists who protect humans from being affected by youmu—and the last surviving member of her spirit hunting clan, their lives become intertwined as Akihito seeks to help Mirai gain the confidence to kill youmu so that she may stop attempting to kill him as practice.

There are also rare cases when a youmu and a human mate may produce an offspring, which is called Half-Youmu (半妖, Han-Yō, "Half-Shade").

Torii entered the first novel in the series into the second Kyoto Animation Award contest in 2011, and it won an honorable mention in the novel category.

The first film, Past (過去篇, Kako-hen), was released on March 14, 2015; its theme song is "Daisy" by Stereo Dive Foundation.

The second film, Future (未来篇, Mirai-hen), was released on April 25, 2015; its theme song is "Aitakatta Sora" (会いたかった空, The Sky I Wanted To Meet) by Chihara.