Takeru Narihira is a lone wolf delinquent who spends his days getting into fights and avoiding connections with others. The only exception is the mysterious girl who appears in his recurring dreams, always saving him from terrifying monsters. On the night of their 1,999th dream meeting, he vows that next time, he will be the one to protect her.

A sudden motorcycle accident sends Takeru tumbling off a cliff, but he does not die. Instead, he awakens in a bizarre, futuristic version of Japan's ancient capital: Denji Heian-kyo. This city is a strange fusion of the Heian era and advanced technology, where mysticism and sorcery are commonplace. To his shock, the girl from his dreams is real and living in this world. Her name is Tsukimiya, though she does not recognize him.

Takeru’s joy at their fated encounter is short-lived. A deadly black mist descends upon the city, petrifying citizens and unleashing monstrous beings known as Oni. The only ones who can fight these creatures are the Onmyoji, sorcerers who pilot powerful magical mecha constructs called shikigami. Takeru discovers he possesses a strange resistance to the mist and awakens a latent power, but it is not enough. In the ensuing chaos, both he and Tsukimiya are killed.

Instead of dying, Takeru reopens his eyes in Denji Heian-kyo, two weeks before the attack. He has traveled back in time, retaining all memories of his previous death. Swearing to defy fate and save Tsukimiya, he seeks out the legendary Abe no Seimei, the leader of the Onmyoji, and begs to be trained in the art of onmyou-jutsu.

As Takeru trains alongside elite Onmyoji like the rigid but talented Atsunaga and the rough-but-pure-hearted Yura, he quickly learns that changing the future is not simple. He dies multiple times, each failure sending him back to his arrival point in the city. With every loop, he uncovers more of Denji Heian-kyo’s dark secrets. The Oni threat is not what it seems, and Abe no Seimei harbors a hidden agenda. The city is revealed to be one of a hundred parallel worlds existing within a pocket dimension, all secretly pitted against each other in a brutal war to evolve their combat technology. Seimei is orchestrating the repeated destruction of these worlds to create a weapon capable of fighting an even greater existential threat. Tsukimiya herself is revealed to be a digitized consciousness experiencing the memories of all her alternate selves.

Throughout the series, Takeru transforms from a simple delinquent into a determined warrior who must confront the moral ambiguity of his allies. The narrative progresses from an initial struggle to survive against monsters, to a battle against the very system of the city. Major arcs include Takeru’s initial training and integration into the Onmyoji ranks, the revelation of the Hundred-Year Pandemonium that resets the world, the shocking truth about the nature of the Oni, and the final confrontation where Takeru must fight not just to save Tsukimiya, but to break the cycle of genocide across the parallel worlds. In the climactic episodes, Takeru faces Abe no Seimei in a battle of ideologies, ultimately striving to forge a new path beyond the endless loops and manufactured destiny.