One day, Shiori, an honor student, visits the house of her shut-in classmate, Yusei. The person who greeted her was her father, Kouji. At first glance, he appears to be a kind, middle-aged man, but he is actually a former yakuza, and has been involved in various underground businesses. Bookmark is sent to Yusei's room and is tied up by Yusei and Kouji. Yusei is also a collaborator of Kouji and others, and he enjoys doing this kind of thing so much that he doesn't go to school.