Caught with a severe fever, Tobio Ikuse, a second-year high school student, was forced to miss his school trip on a luxury liner with his schoolmates. Four days later, Tobio hears the news about the disappearance of the teachers and second-year students from his school, and it devastates him. A few months after the incident, Tobio, having slowly recovered from the sudden loss of his friends, accidentally encounters one of them again. Chasing after his missing friend Sasaki, Tobio finally manages to catch up to him but is shocked to see Sasaki alongside a Lizard Monster who attacks him together with the monster. Cornered by the monster, Tobio is rescued by another survivor of the missing luxury liner, Natsume Minagawa, who then hands him an egg before parting ways. Later that night, Tobio finds himself under attack, again, this time by two people with monsters. Amidst all the ensuing chaos, out of Tobio's shadow, came out a Puppy that rescues him. After fending off the two monsters, Natsume once again approaches Tobio before finally deciding to take him to their hideout.

Hyoudou Issei is an ordinary yet lecherous high-school student who is killed by his girlfriend, Amano Yuuma, during their first date. Yuma is revealed to be a fallen angel named Reinare who was sent on a mission to eliminate divine weapons. Issei is later reincarnated as a demon by his senpai Rias Gremory; in return, he will serve under Rias to fight against the fallen angels.