Hidden within Kyoto is the “mirror capital” where youkai and humans live together and no one ever dies. It is protected by three people—Kurama, Yase, and Myoue—as they await the return of their parents, Myoue Jonin and Koto. One day, a young girl named Koto (same name, different spelling) and her little brothers A and Un wander into the city from a different dimension looking for her mother. A mecha-geek with red eyes, it’s not immediately clear whether she’s human or youkai. Her arrival upsets the long peace that had ruled the city.
It’s only after their plan is thoroughly decimated by the infamous Underground Student Council that the motley crew find their freedom abruptly taken from them, as they are thrown into the school’s prison with the sentence of an entire month as punishment. Thus begins the tale of the boys’ harsh lives in Prison School, a righteous struggle that will ultimately test the bonds of friendship and perverted brotherhood.