About Daigo as a wanderer who claims to be a genius and strongly believes in justice. One day, at a playground, he sees a boy, Hiroki Miura, being bullied by other kids. Daigo attempts to save the day. Then, the kids' homeroom teacher, Maria Kagami, explains to Daigo about how Hiroki is a timid boy who never speaks his mind, which has warranted him to be picked on. To help Hiroki, Daigo gets a job as a special teacher at his school. As Daigo tries to get closer to Hiroki, he learns that while Hiroki may be reserved, he is actually passionate about and confident in himself when it comes to playing the game of Cardfight!! Vanguard.