Mo Qihan, the second prince of the demon world, who possesses unparalleled defensive magic, unparalleled healing magic, and offensive magic that is only lower than three people, was kicked out of the palace by his father when he was 6 years old. One night after wandering in the demon world for several years, he met the guardian spirit of the Netherworld Library, Youshuang. He fell in love with the black cat at the first sight, and agreed to Youshuang's request to become the master of the Netherworld Library. Later, after I learned that Yousui could transform into a human form, I fell in love with Youshu even more. But Han, who doesn't know You Gu's feelings, is in painful unrequited love...