Amidst the destruction of the city, there exists a game solely meant for children known as "Otokoyo". This game involves the traditional hide-and-seek. However, peculiarly, when the kids participate in this game, they end up disappearing into the darkness amongst the ruined buildings. Our tale begins when a boy sets off into the enigmatic world of Otokoyo, on a quest to locate his sister who has vanished into it.