Shu pediu as contas de seu emprego antigo para começar sua própria empresa, mas está sem um tostão. Ele sobrevive graças à caridade de Akino, sua antiga namorada e ex-colega de trabalho, e de Kisara, uma jovem colegial que trabalha em sua companhia. Sua sorte muda quando surge um novo trabalho para exterminar uma Ameaça-D de nível médio, e que misteriosamente lhe desperta o interesse.

Kisara ganha uma chave para a casa de Shu, o que, para ela, é um fortíssimo sinal de que eles estão, agora, morando juntos. Infelizmente, Shu é péssimo com finanças, e não tem um tostão para sobreviver um mês inteiro. Sua única solução é pedir trabalho à sua ex-namorada, que o aloca em uma missão para evitar um ataque terrorista num evento político.

Uma nova Ameaça-D surge em uma fábrica, mas após engolir um valioso núcleo energético, nem mesmo os preços competitivos de Shu são argumento o bastante para lhe conceder a concorrência. Quando Ayano se encontra em apuros, Shu e Kisara entram em ação, revelando a verdade por trás do elo entre os dois.

Mortes suspeitas em uma briga de gangues indicam um possível envolvimento de Possuídos e Demônios. Ayano lança mão de suas conexões governamentais para investigar esse caso por conta própria, o que a coloca numa posição delicada e antagonística contra Shu, o maior interessado em investigar os demônios para saciar suas motivações pessoais.

A Possuída escolhe Ayano como seu alvo, e devido a seus poderes, obriga a jovem a se manter em isolamento de quarentena para não colocar os outros em risco. Enquanto isso, o inspetor Mikami investiga quem está vazando informações confidenciais de Ameaças-D para o Shu, sob a suspeita de que o responsável por esses vazamentos tenha intenções escusas e ligações com demônios.

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.

The memories Kisara stole from the demon revealed the truth behind the attack on Shu and his family 12 years ago. Asmodeus, a demon who changes their appearance, transcends time, has traveled around the world for over a century, created Demonically Possessed and inflicted harm upon humans. Like a hazardous calamity, Asmodeus is the bitter enemy that the Celestial Abbey hunts down. And the true enemy that killed Shu's father and took his sister. Moreover, this demon is supposedly involved in the origin of Orgonium, the root of Bayron City's success. It's now a battle for the city, no, for the fate of humanity and demons alike...

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?