Jin, Akira, and Lena are three childhood friends who are one day tasked with cleaning up an old wing of the local library. As they organize the books, the children find an old tome titled "The Great Demon Encyclopedia," containing drawings and descriptions of multiple demons. Out of curiosity, the trio decides to recite a spell contained inside the book, and finds that time has stopped in the human world for everyone except them. They see a vision of a young girl called Ami, begging them to "save Valhalla." After a flash of light, they find themselves in the world of Valhalla, face-to-face with Ami.