There was once a man who was summoned to another world, and saved it. Of course, he became too popular there, and turned into an isekai-normie. However, that man fell into a "trap" and was forcibly returned to his original world. Moreover, he had to start over as a baby! This is the story of the way-too-fantastic ex-hero who lived as a gloomy high-schooler, as he gets summoned once again to that same other world in a very unexpected development!

Under the belief that the omnipotent god Mitama will come to save him, Yukito Urabe participates in a ritual to become the new leader of his father's cult. But when the boy drowns during the ritual, he wishes to be reborn in a world without gods or religion. Reawakening in a completely different world devoid of spirituality. He meets a deviant girl named Aruaru, who introduces him to her village. However, his idyllic image of this world's society is short-lived when he witnesses a public group suicide and learns of the country's end-of-life system: at any moment, the government may order any citizen to die. Aruaru and her sister are forcibly taken for execution soon after, prompting Yukito to rush to their rescue—but to no avail. In a moment of desperation, Yukito recalls his father's teachings and utters a prayer for Mitama to save them. Seemingly answering his call, a little girl descends from the sky and annihilates everyone who harmed Yukito and his friends. To Yukito's surprise, the girl introduces herself as Mitama.

High schooler Hayase Nagatoro loves to spend her free time doing one thing, and that is to bully her Senpai! After Nagatoro and her friends stumble upon the aspiring artist's drawings, they find enjoyment in mercilessly bullying the timid Senpai. Nagatoro resolves to continue her cruel game and visits him daily so that she can force Senpai into doing whatever interests her at the time, especially if it makes him uncomfortable. Slightly aroused by and somewhat fearful of Nagatoro, Senpai is constantly roped into her antics as his interests, hobbies, appearance, and even personality are used against him as she entertains herself at his expense. As time goes on, Senpai realizes that he doesn't dislike Nagatoro's presence, and the two of them develop an uneasy friendship as one patiently puts up with the antics of the other.

With the appearance of a mysterious virus that kills everyone above the age of 13, mankind becomes enslaved by previously hidden, power-hungry vampires who emerge in order to subjugate society with the promise of protecting the survivors, in exchange for donations of their blood. Among these survivors are Yuuichirou and Mikaela Hyakuya, two young boys who are taken captive from an orphanage, along with other children whom they consider family. Discontent with being treated like livestock under the vampires' cruel reign, Mikaela hatches a rebellious escape plan that is ultimately doomed to fail. The only survivor to come out on the other side is Yuuichirou, who is found by the Moon Demon Company, a military unit dedicated to exterminating the vampires in Japan. Many years later, now a member of the Japanese Imperial Demon Army, Yuuichirou is determined to take revenge on the creatures that slaughtered his family, but at what cost? Owari no Seraph is a post-apocalyptic supernatural shounen anime that follows a young man's search for retribution, all the while battling for friendship and loyalty against seemingly impossible odds. [Written by MAL Rewrite]