Issei was walking home to celebrate for passing the entrance exam in Kuoh Academy until he saw a girl being attacked by a devil in an alleyway. He managed to save the girl by awakening his sacred gear and defeating the demon but was left bleeding in the ground until a portal opened up, sucking him into a new world. (IsseixHarem)

After The Beast of The Apocalypse Trihexa has been unsealed, the world was nearing its end but was saved from its destruction by non-other than Hyoudou Issei who defeated the beast. But in his final moments Hyoudou Issei, suddenly found himself under a shattered moon.