Shu es un joven que decidió abandonar la empresa de contramedidas contra Incidentes D en la que trabajaba y formar la suya propia. Aunque lo hizo con mucha convicción y no se arrepiente, lo cierto es que apenas tiene trabajo como para poder ganarse la vida. Por suerte para él, a su lado tiene a alguien muy especial.

Aunque Bayron City es un paraíso a ojos de su alcalde y el exterior, la realidad de su interior es muy diferente, con demonios que campan a sus anchas, sueños rotos y gente obligada a vivir en la parte clandestina de la sociedad. Pero para Shu la preocupación es tener algo que llevarse a la boca.

A large-scale power outage hits the heart of Bayron City's Downtown. It was a D Hazard caused by a giant caterpillar-shaped Demonically Possessed. With detailed information on Demon Possession that he obtained from somewhere, Shu decides that this is a job that "must be done" and uses Ayano for insider info to pull strings. As Shu's company is about to win the bidding war at a discount, significant new information about the Demonically Possessed is revealed. Those who took the situation most seriously decided to suspend bidding and expelled Shu from hazard prevention measures.

Although the Demon Possession that caused the large-scale power outage was exterminated, the problem remains unresolved as the power shortage in Bayron City continues. Meanwhile, Kisara witnesses a secret meeting between Ayano and a mysterious man. To confirm Shu's feelings, she uploads a photo of the scene to social media. Successfully lured by Kisara, Shu rushes Ayano and the mysterious man – an Anti-Demon Bureau detective named Tetsuya Mikami – and, in an unexpected flow of events, realizes that Ayano was hiding her intentions and planning behind his back. An unparalleled sense of tension lofted between the two.

Ayano has become a target for Demon Possession. Although Ayano narrowly escapes her first attack with Kisara's help, she begins a life of isolation to hide her Demonically Possessed eyes. Meanwhile, Shu is offered a chance to work together by Mikami, who is familiar with Shu's past. On board with the discussion, Shu discloses information to Mikami and his team at the Anti-Demon Bureau. They are trying to determine the identity of a Demon Agent producing Demon Possessions one after another. They step into a certain dwelling, but...

Kisara keeps her distance from Shu's apartment since she saw the memories of Shu and Ayano's time in the warehouse. Reversely, happy for Kisara's absence, Ayano takes good care of Shu as she relishes feeling like they are lovers. Kisara, who checks out the situation via social media, is getting increasingly more annoyed. But behind the scenes of this love triangle is a storm of violence that blows into Bayron City. Just who is this blonde beauty who appears out of nowhere wearing a nun's habit? And what is her goal as she repeatedly commits senseless acts of brutality by invading scene after scene of Demon Hazards?

The Celestial Abbey — — Publicly seen as advocates for philanthropy, they are the world's largest Exorcist group. The identity of the mysterious nun who suddenly appeared in Bayron City is one of the most dangerous persons in the Celestial Abbey, Sharon Holygrail. Known as "Sharon the Chaste," she is one of the 12 remarkable Exorcists called "Living Relic." The Anti-Demon Bureau faces a growing sense of danger with the emergence of a new power that could shake the framework of their organization. Meanwhile, as Shu is injured and sleeping, Kisara tells Ayano about the events atop the snowy mountain that led to the intertwined fate of her, Shu and Sharon.

Thanks to Shu and Kisara's efforts, the Anti-Demon Bureau successfully detained Sharon, and peace returned anew to Bayron City. With his companions Mikami and Miles, Shu once again pursues the truth behind the continuous D Hazards that have been manipulated by some entity's volition. They continue compiling deductions to pin down the identity of the "True Agent," who they presume is lurking behind Junya Hanamura, the murdered Demon Agent. At the pinnacle of their deductions, Mikami reaches a certain conclusion and tries to contact Shu in a hurry...

The evidence gathered points to the real Demon Agent. And it was Shu's most trusted person. As the Police Agency and top officials of AAA continue to strategize and negotiate, Shu confronts the true criminal alone. Why? For what reason? All words and all questions are in vain when the real criminal transforms into a vicious demon right before his eyes. Its power is so overwhelming that, even with AAA's fighting strength concentrated, there is no damage. At this rate, all the people Shu cares for will be hurt. With raging anger and profound sadness in his heart, Shu decides to offer all his precious memories to Kisara. All for the sake of defeating the demon.

In the middle of the battle against the demons on the artificial island, Kisara was stabbed by a spear before Shu's eyes. Saved by Ayano and Sharon, Shu helplessly evacuates from the fight when he sees the person he's been looking for—his younger sister, Kanna. Then, humanity's desperate resistance, backed by the focused efforts of Bayron City, proves futile as the extent of the large-scale disasters caused by Kanna, now certified as an S-Class demon, expand. Kisara collapsed due to her injuries and lost consciousness as her vitals disappeared. Unable to move, Shu just stares at her. And as if responding to his heart, Kisara gently awakens.

Shu is always in trouble with money but manages to live thanks to the support given by his ex-girlfriend Ayano and the ever-devoted Kisara. The same goes for his job as a PMC. With Ayano's cooperation, Shu won a bidding war at a discounted price to participate in the "D Hazard" prevention operation that occurred in a casino. At the scene of the demon extermination, he planned to rely on Kisara's demonic powers, but she is nowhere to be found due to the money situation ruining her mood. As a last resort, he works with Ayano's leading, major PMC and confronts the Demon, but bit by bit, the battle becomes something suspicious...

In the chaotic mix of humans and demons, Shu and Kanna meet in a violent clash worthy of their 12-year-gap as various motives intermingle in a battle engulfed by a sea of flames. Ayano, Sharon and Kisara try to somehow intervene in the siblings' fight that puts the fate of the world at stake. The only way to stop a powerful demon is with the power of a demon. Shu and Kisara join hands once again. Will the accumulation of their feelings reach Kanna, embroiled in her rage? At the end of the battle, what future lies ahead for humans and demons alike?