The book tells the story of a delivery boy who rarely went out to play after his sister finished her college entrance examination, but was attacked by unknown creatures. He was later rescued by members of a mysterious organization, and finally joined the organization to save his life. After joining, he discovered that there were different strange creatures everywhere behind this seemingly peaceful world. Little did he know that there were more unknown events waiting for him, and everything about him was actually planned by his own sister, who also had an unknown secret.
