The episode opens in the Third Library, in a room behind the curtain where Rem is visiting his father. His father asks him what is taking him so long. Rem tries to explain but his father cuts him off by asking him if he has forgotten his pride as a devil in the human world and warns him that he will atone for his betrayal with death. This shocks Rem who protests that he swore his loyalty towards him, only to be rebuked when he is sent flying across the room. His father tells him that all he desires is the Forbidden Grimoire before grabbing Rem and choking him. He also tells Rem that if he wishes to carry on the Arlond name, he has to make the grimoire his. In the Third Library, Urie expresses his relief that he isn't the Arlond heir and Mage suggests that they go after Ritsuka while Rem is occupied with his father. Urie and Mage bicker over Ritsuka until they notice that Shiki went ahead prompting them to go after him. After they are gone, Rem reappears in the Third Library, battered and bruised after being thrashed about by his father. He staggers towards the sofa and lies down to rest. Roen watches this from beneath the table before exiting the library through a passage behind a curtain.
Meanwhile, Ritsuka and Azuna are at a department store shopping for materials and equipment for the Cultural Festival. Just as Ritsuka is about to hurry off to fetch the last item they need, Azuna grabs her hand. She reminds her that she promised not to go off on her own, but Ritsuka assures her that a little far won't hurt. Azuna firmly tells her to not be overconfident, because who knows where or when the devils will show up and Ritsuka sighs that Azuna is just like her brother. They soon finish their shopping and as they make their way to the checkout counter, Azuna sighs that they won't be able to carry everything back and mutters how the boys in their class don't lift a finger for the Cultural Festival. Ritsuka suggests that they have their shopping shipped to the school since preparations for the festival begins tomorrow. Azuna agrees and goes to check out the items. She instructs Ritsuka to wait for her and to not wander off on her own no matter what. Ritsuka complies and it is not long until she spots Roen hiding among the Pomeranian plush toys. As she wonders why he's here, he suddenly scurries off prompting Ritsuka to pursue him.
She eventually follows Roen to a nearby park where she finds him hiding in the bushes. She pulls him out and as they sit down on a park bench, she learns from his name tag that his name really is Roen. Ritsuka sternly asks Roen if he came out here by himself saying, that he could have easily gotten lost. Roen responds by jumping onto Ritsuka's lap and allows her to stroke his paws. She tells him that she'll take him back to Rem, but Roen suddenly starts sniffing her and licking her neck. There is a sudden puff of smoke and Roen transforms into the mysterious person Ritsuka encountered at the Third Library (see Episode 4). This startles Ritsuka and so does Roen when he realizes that he transformed. He tells Ritsuka that this is her second time seeing him in his human form and introduces himself as Roen. Ritsuka screams in shock and pushes Roen away just as her phone rings. She is about to answer her phone until Roen intercepts and he asks Ritsuka if she is interested in knowing more about the Forbidden Grimoire. Hearing this, Ritsuka becomes cautious of Roen and she asks him if he is a devil. Roen tells her that he is a Cerberus who serves a fiend known as Maksis. He then tells Ritsuka to come with him if she wants to know about the grimoire and assures her that he won't force her if she doesn't want to.
Ritsuka becomes convinced and answers her phone. She lies to Azuna that there was something else she had to do, but she will go home straight away afterward. After she hangs up, Roen comments on how naughty she is to lie to her best friend. Elsewhere, Azuna is walking through the neighborhood until Shiki appears and flies down towards her. This startles Azuna as Shiki rushes to hide behind her just as Urie and Mage appear before them. Mage tells Shiki that he has got a lot of nerve getting the jump on the devils and he punches an electricity pole right next to him. The electricity pole falls towards Shiki and Azuna until Urie uses his rose whip to discard the pole seconds before it could crush them. Urie asks Shiki if he really wanted to be caressed by his thorned rose whip but Shiki tells him that its a misunderstanding. He explains he was trying to help them by leading them to Azuna because she knows where Ritsuka is. Meanwhile, Rem awakens in the Third Library and he notices the curtain obscuring the passage is slightly opened, causing him to become suspicious. At the same time, Ritsuka is following Roen for a while through the passage and she asks him what the Forbidden Grimoire is, saying that she's heard it is a book.
Roen doesn't answer her question and he suddenly senses a presence. He tells Ritsuka to hurry, but she becomes infuriated and makes it clear to Roen that she will stop following him if he won't tell her what the Forbidden Grimoire is. Hearing this, Roen becomes impressed and he compliments Ritsuka telling her that her stubborn and courageous attitude proves that she is indeed her father's daughter. This confuses Ritsuka and Roen explains that his master, Lord Maksis is in need of the grimoire in order to restore his failing power. Ritsuka asks him if the grimoire contains a method for reviving the devils, but Roen tells her that she is wrong. He explains that the grimoire itself is the great power which will allow anyone who has it in their possession to control the world. His eyes suddenly glow which blinds Ritsuka and he disappears. He then reappears behind Ritsuka and licks her neck telling her that she is emitting a lovely fragrance, but she shoves him away. The ring of the Sacred Lux glows and repels Roen away from her. Roen calmly picks himself up and reapproaches Ritsuka, but she orders him to stay back.
Roen approaches her, saying that everything of her is in the scent of the grimoire. He eventually corners Ritsuka and reveals to her that she is the Forbidden Grimoire herself, and the scent she has been emitting is the proof. This shocks Ritsuka and Roen tells her that whoever obtains her will have the power to open the gates of Gehenna and rule the world. He then offers to help Ritsuka to remove the grimoire from her, and through his song 'I am a Faithful Dog' ("Watakushi wa chūjitsuna inu desu" ワタクシは忠実な犬です), he convinces her saying that the war between the devils and vampires will end and Ritsuka can return to her normal life with her family. Roen tells Ritsuka that Lord Maksis is his all and once his master has the grimoire, she and her family will be of less interest to the devils and vampires. Meanwhile, Azuna uses her rosario and holy water to protect herself from the student council. Mage tells Azuna to stop fooling around but she orders them to stay back. Urie pressurizes Azuna by informing her that she is out of holy water and Shiki orders her to tell them where Ritsuka is unless she wants to know the consequences if she doesn't. Lindo suddenly appears forcing the student council to retreat. After Shiki uses his wings to teleport himself, Urie and Mage away, Azuna sinks onto the ground with relief as Lindo rushes up to her. He asks where his sister is, which startles Azuna who claims that Ritsuka should be home by now.
Meanwhile, Rem is wandering through the passage when he overhears Ritsuka telling Roen that she cannot believe that she is Forbidden Grimoire. Roen tells Ritsuka that the grimoire will activate on her seventeenth birthday and will last until the night of the next full moon. Hearing this, Ritsuka remembers her brother telling her that both he and their mother were planning to reveal everything before her seventeenth birthday (see Episode 4). This shocks Ritsuka and Roen reads her mind, telling her that both her mother and brother already knew the fact that she is the Forbidden Grimoire. He compliments Lindo for being a good liar but agrees that Lindo only kept quiet about it in order to protect the grimoire nurturing inside her. Unbeknownst to him and Ritsuka, Rem is now nearby and has overheard everything. Roen then tells Ritsuka to not be afraid as he opens a portal to a bedroom in the devil world. He tells her that the grimoire is all he needs and suggests to Ritsuka to sleep in this bedroom until her seventeenth birthday. Roen then orders Ritsuka to discard the ring of the Sacred Lux in order to proceed.
Ritsuka hesitantly prepares to remove her ring, but Rem appears and he grabs her hand, stopping her. This startles Ritsuka and Rem firmly tells her that she can't go. She quietly asks why he is stopping her and is shocked to see him covered in scratches. Roen sighs and tells Rem that if he is not careful, he might be torn from limb to limb next time he meets with his father. Rem then turns his attention to Roen as he angrily tells him that he cannot believe that Roen was still working for Lord Maksis. He questions him about his obligations towards the Arlond family, which Roen scoffs at causing Rem to attack him. Roen dodges Rem's attack and grabs Ritsuka, taking her hostage. Rem orders Roen to let her go, but Roen threatens to rip her heart out in front of him. He uses his claws to rip her uniform before reaching into her blouse to slowly stab her skin. Ritsuka screams at him to stop and repels him with the ring of the Sacred Lux. She sinks onto her knees with relief just as Lindo appears. He rushes over to his sister and notices that she is bleeding from Roen's injury and because Rem is there, Lindo immediately assumes that it was Rem who injured Ritsuka.
He flies into a rage as he approaches Rem, but Ritsuka rushes in front of her brother, in a desperate attempt to clear up the misunderstanding. She tries to tell Lindo that Rem was helping her, but Lindo suddenly transforms and he orders Rem to stay away from his sister. Elsewhere, Roen is hiding behind a pillar and is startled when he sees Lindo's transformation. This shocks Ritsuka and Lindo uses his supernatural ability to get past her in order to confront Rem. Lindo raises his fist towards Rem, but Ritsuka grabs him from behind and Lindo roughly shoves her away. Ritsuka is sent flying across the floor before getting sprawled on the ground after landing hard. As she picks herself up, Rem rushes over to help her until Lindo appears behind him. Lindo raises his hand which has become claws and prepares to bring it down on Rem. Rem shields Ritsuka, but she suddenly covers him causing Lindo to scratch her across the cheek. Lindo prepares to attack Rem again, but Rem manages to block Lindo's attack with his hands and they end up wrestling. Feeling that Lindo is gaining the upper hand, Rem desperately asks him if he'll destroy Ritsuka too. Hearing this, Lindo realizes what he is doing and calms down, transforming back to his usual self.
As he frets over what he has done, Ritsuka rushes up to her brother and embraces him, assuring Lindo that everything will be fine. Rem quietly gets up and leaves, Ritsuka staring after him. They are teleported back to the park where Ritsuka met Roen. Elsewhere, Roen is hiding behind a tree and he vows that he will not give up and will eventually obtain the grimoire for Lord Maksis as he disappears. Later that night, Lindo apologizes to Ritsuka for what he did to her. Ritsuka quietly asks him if he knew about her being the grimoire all this time. Lindo confirms this, having had been told by their grandfather and their mother also knew about it as well. He asks her if she remembers wearing the pendant containing potpourri and reveals that the potpourri was specially grown by Maria which completely masked the scent of the grimoire. Lindo also tells Ritsuka that the first time the grimoire surfaced was a year ago when she learned of their grandfather's death. The grimoire reacted to her emotions which caused its fragrance to be emitted from her body, and the potpourri wasn't even able to hide it. Although it was for a moment, it was just enough for the devils and the vampires to realize the grimoire's existence, leading to their family being targeted.
Hearing this, Ritsuka exclaims that it's all her fault that their mother was kidnapped, but Lindo tells her not to think like that. She tearfully tells him that she wants all this to end and decides to hand over her power in order to end the conflict, but Lindo tells her that she cannot do that. When Ritsuka asks why, Lindo tells her that if she does, it will be the end of her. This shocks Ritsuka and Lindo tells her to trust him, because he is determined to keep her safe even if it costs him his life. Going to bed, Ritsuka quietly says that everyone is hiding things from her as she recalls what happened to her brother earlier today. She wonders out loud who she can trust now and can no longer differentiate between truth and lies. Meanwhile, in his mansion, Rem places a shōgi piece on his board while declaring checkmate. He glances at the injuries Lindo inflicted on him as he recalls how he stopped Ritsuka from putting herself in a deep slumber. The episode ends here.