The beginning of the arc starts with a user logging onto he chat room under the name Saika and starts spamming messages about love and the desire to find someone strong. After it logs off, users Kanra, Setton, and TarouTanaka log on to discuss the user. While they are conversing, Kanra brings up rumors involving a demon blade.
Elsewhere, a man is being chased by the Black Rider. Celty corners the man in an attempt to retrieve the stolen money from him when she is suddenly stabbed in the arm with a blade. Her assailant attempts to decapitate her, but being headless, this does not hinder Celty. She turns around to catch her attacker, but the person swiftly runs away. The man, seeing Celty's headless form, also scurries away.
The next day Anri Sonohara and Mikado Ryuugamine are performing their class responsibilities. Mikado offers to turn in her notebook for her, but Anri declines and leaves the classroom where she is approached by one of the teachers Takashi Nasujima. Takashi attempts to sexually harass Anri when he is stopped by Masaomi Kida who had overheard and recorded the whole conversation and threatens to give the evidence to the other teachers, effectively scaring Takashi away. Masaomi then warns Anri about him and rumors of a similar incident involving a girl named Haruna Niekawa, who ended up transferring schools because of him. Mikado encounters the two in the hallway, and they all walk home with each other.
The friends split up when they are getting closer to their homes. Anri walks the way to her house when she gets kicked down by one of the ganguro girls. They harass Anri for relying on others and using them. As Anri is being bullied by them, she enters her frame room as an escape when she notices a shadow behind the girls. The figure brandishes a sword and attacks the girls before running away.
In the chat Kanra mentions the recent slashing with the high schoolers that got slashed. TarouTanaka becomes worried for his friend when he hears the news.
Elsewhere, Shuuji Niekawa is trying to find out the strongest man in Ikebukuro for an article in Tokyo Warrior. He goes around the city, interviewing various people for information, starting with Simon and Denis at Russia Sushi. He is referred to multiple people: Haruya Shiki, Izaya Orihara, and the Black Rider. But ultimately he is directed towards Shizuo Heiwajima. Shuuji meets with Shizuo's superior, Tom Tanaka, who warns Shuuji not to irritate Shizuo in anyway. When Shizuo comes into the room, Shuuji has a hard time believing him to be the strongest based off his looks and quiet demeanor. Disregarding Tom's warning, Shuuji purposefully instigates Shizuo to make him lose his temper, resulting him in getting blown across the room. After Shizuo's display of his strength, Shuuji begins to head home, excited to write the article, unaware of a figure stalking up behind him.
Later in the chatroom, user Kanra talks about Shuuji Niekawa being slashed, concerning Setton. Saika logs on not long after and begins to spam the chat, proclaiming it has found Shizuo and its love for him. Kanra mentions how this user logs on in various points in Ikebukuro, indicating the user was a local resident who may be logging on from different IP addresses. TarouTanaka guesses the user Saika is related to the Slasher when it mentions wanting to cut people.
Elsewhere in Saburo Togusa's van Kyouhei Katoda is watching the news on the recent slashings. He takes note on the raise in number of victims as well as seeing more Yellow Scarves members on the streets, worrying they would become a problem soon. As Kyouhei is discussing the slashings, everyone in the van receive a message on the Dollars website, which announced that a Dollars member has been attacked and someone needs information on the Slasher.
In Celty's apartment, Celty tells Shinra about the Dollars message and Shinra mentions a similar case that took place five years prior. He mentions that although there were less victims there were cases of people getting killed by the assailant. When Celty comments how the culprit was never found, Shinra informs her of the demon blade Saika that also shares the name of the spam user in the chatroom she's a part of. Celty goes out to see Shizuo, worried as his name was mentioned by the Saika user in the chatroom and wondering how he is correlated with the slashing incidents.
Celty finds Shizuo in a park where he begins to talk about his abnormal strength and how he acquired it. Shizuo reveals he first awakened his superhuman strength over a fight he had with his brother Kasuka when he was in third grade. He goes on listing many more incidents growing up where he would lose his temper and get in fights, as well as break his body in the process from the excursion. After Shizuo talks about all his worries and frustrations he has been dealing with, he then asks Celty what she came to see him for. Celty explains the slashing incidents taking place across the city and how they may possibly correlate with the user Saika on the internet. When she mentions his name popping up in the chat the same night Shuuji Niekawa got attacked, Shizuo initially thinks Celty was suspecting him. Celty denies it, aware the slashings were not something Shizuo would do. She brings up the message on the Dollars forum, but Shizuo is not interested in finding the Slasher until Celty admits she was attacked by the culprit as well. Upon hearing that, Shizuo is willing to help Celty find the Slasher, but Celty convinces him they need more time to collect information on the slasher before searching for them, allowing her to visit Izaya in Shinjuku to learn more about Saika.
Celty rushes home after her findings and retells what she had learned to Shinra. Celty is unsure how she could track Saika down when she could not sense its presence, unlike how she usually can with supernatural beings. Shinra tells her he had looked at the chatroom earlier and suggests her to look at the backlogs. Celty finds the user Saika having mentioned they would cut someone again the next day and that they were searching for Shizuo.
Elsewhere, Anri has a dream of her parents, who had died five years ago, and wakes up alone in her apartment. She proceeds to go through her daily routine, but after classes she is approached by Takashi again. Takashi once more tries to make a move on her, using the incident with the bullies as a way of leverage, but Anri turns that back on him, suggesting she could just transfer to a different school like Haruna Niekawa did. This takes him off guard, and Anri is able to walk away without issue.
After escaping Takashi, Anri begins to wander around the city in a vain attempt of finding anything on Haruna for her to use against Takashi. She ventures around the city aimlessly when she is stopped by two cops. One of the officers, Kuzuhara, offers to escort her home, but they receive a call they had to respond to. Anri begins to walk home, unaware of someone with red eyes tailing behind her.
Meanwhile Celty meets up with Shizuo and tells him about the demon blade. Shizuo agrees to go with her without hesitation or questions. The two get on Celty's bike and begin to ride around the city in search of the slasher. Elsewhere, Erika Karisawa and Walker Yumasaki are having an argument over their takes on a book. Kyouhei intervenes to try and calm them down and change the topic, though does not accomplish much. He turns to look out the window and notices they were near the spot the Raira high school girls were attacked at when he sees Anri walking by herself and a man with red eyes following her. The man approaches Anri and swings his weapon in the air, posed to strike. Seeing that, Kyouhei demands Saburo to run over the man. Saburo floors it, slamming into the slasher.
The vang gang get out of the vehicle to check and see if the man is alive when the slasher stands up and makes another attempt to attack Anri. Kyouhei snaps into action and tries to intervene, but the slasher swings his blade at him. When Kyouhei distances himself from him, the man turns back to Anri. Just before he could reach her, Celty arrives with Shizuo and slams right into the slasher. Kyouhei blurts out Shizuo's name when he sees him, which catches the slasher's full attention. Celty deduces from the man's feminine speech and bizarre mannerisms that he must be under Saika's control. Celty goes over to guard Anri and check her condition while Shizuo faces off the slasher. Shizuo turns to Kyouhei and states he would take the door before the other could respond. He pulls off the van's side door and uses it as a shield as he rushes straight at the slasher, slamming him against the wall.
Celty plans to tie him up and see if he was the real slasher or just the puppet, but the man wakes up and lunges for Anri one last time. Celty blocks the attack with her body, getting stabbed in the chest. She grabs the slasher and flips him over. She takes away his knife and subdues him. Once the man was unconscious, Celty wraps the kitchen knife he had with shadows, planning to take it to Shinra to dispose of it. Shizuo doesn't feel satisfied, despite the slasher being taken care of, and heads to Shinjuku to find Izaya, suspicious. After Shizuo leaves, Celty explains the demon blade to Kyouhei and his friends. Kyouhei put the slasher in their van and take off. Once they leave, Anri approaches Celty and asks her to tell her everything that has been going on in the city lately. After providing the general explanation, Anri then asks who Celty is and why she does not have a head, inviting Celty to come to her place to talk more in depth.
Celty arrives home with the blade she took from the slasher and shows it to Shinra. Celty wonders why Saika is attacking people out of its so-called love, prompting Shinra to explain Saika's nature and motivations. As they are discussing Saika, Shinra notices markings on the hilt of the kitchen knife, showing the item was made in the modern era. They realize the knife Celty secured was not the real Saika, meaning the man earlier was just a puppet and the real slasher is still at large. Celty tells Shinra the man - Shuuji Niekawa - was also a victim of the slashing attacks, so he was not just copying the slasher.
Meanwhile, Anri is at home thinking over what Celty had told her and regretting not bringing up something when she heard a doorbell. Worried that Takashi had found her home, Anri goes to check who it is through the peephole only to see a woman there she did not recognize. Anri opens the door to greet her, who introduces herself as Haruna Niekawa. Anri and Haruna sit down to talk about Takashi. Anri quickly dispels that she does not like Takashi, but while Haruna is aware of that truth she refuses to forgive anyone who even catches Takashi's eye. Haruna admits she tried to have her father take Anri out, but since he failed she had come to take care of her personally. She pulls out a knife, stating she would have her Saika take hold of her.
Back in Celty's apartment, Celty goes to check the chatroom and sees Saika's recent spam of messages, demanding to see Shizuo again and all their sisters to gather at South Ikebukuro Park. Upon seeing the messages, Celty rushes over to where Shizuo is in Shinjuku, where he is trying to break into Izaya's apartment in an attempt to interrogate Izaya about the slashing incidents and how he is involved. Celty manages to arrive just as the two are about to fight and shows Shizuo the recent messages in the chatroom. Shizuo goes with Celty to the park mentioned in the chat shortly afterwards.
Meanwhile in Anri's apartment, Haruna mocks Anri for latching onto Mika and her two new friends as well as other incidents in her past she had researched about including her parent's deaths five years ago. Anri remains silent until Haruna comments how easy her life is to live by latching onto people. Anri retaliates, saying how hard it is to make sure the people she chooses to latch onto don't drive her off, leading her to insult Takashi. Enraged, Haruna decides to kill her and is about to thrust her knife at her when the doorbell rang. Haruna goes to check who has arrived, preparing to cut up whoever it is in front of Anri to scare her, only to see Takashi at the doorstep. When Takashi sees Haruna, he shrieks and runs off in terror. Haruna goes to chase after him but not before sending in her Saika children to take care of Anri.
Elsewhere, Celty and Shizuo arrive at the park. It is initially empty until a crowd of people emerge from the shadows and surround them. One of the Saika children, wearing a Raira uniform, addresses Shizuo, pleased to meet him. Shizuo asks the girl why they were all so interested in him. The girl replies that it was because of his natural human strength. This answer greatly amuses Shizuo as he bursts out into laughter. In this moment, Shizuo decides he doesn't have to hold back and can use his full power as he would like to for the first time in his life. He then lunges at all the Saika children, fighting them bare-handed as Celty watches in awe. In the midst of the fighting, the Saika children abruptly look over in one direction, sensing the original Saika. Shizuo and Celty take notice, and Shizuo prompts Celty to head over there to see what is going on. Celty agrees, but before she leaves she makes gloves with her shadows for Shizuo to wear so he can touch the blades without getting cut.
Meanwhile, Haruna corners Takashi a little ways from the park. She is about to stab Takashi with her Saika when Anri intervenes. Shocked that Anri managed to get away from her children unscratched, Haruna demands how she got out. However, Anri pleads for her to stop. When Haruna tries to dismiss her, saying she had no business in this, Anri replies that it does have to do with her. Although confused what Anri is talking about and her behavior, Haruna decides to dismiss it and just kill her. She charges at Anri and goes to strike. Anri blocks her blade with her right arm, yet the impact causes a metallic ring. Haruna sees a metallic shine in her sleeve and realizes who Anri is. Anri pulls out a katana from her right wrist, revealing the original Saika.
It is revealed that Anri's family was not killed by a burglar as she claimed to in the police report. One day, her father tried to strangle her to death. Just as Anri was losing consciousness, her mother Sayaka had decapitated her husband's head before promptly using the sword she wielded to commit suicide. Anri had touched the fallen sword and ever since then she had become Saika's new host.
In the present, Anri slides her sword against Haruna's neck before she could reach for her knife. She requests Haruna once more to tell the other Saika children to stop attacking people. When Haruna refuses, Anri presses her sword against Haruna's skin, allowing her to hear her Saika's voice before pulling away when Haruna is overwhelmed by it. Haruna grabs her knife when she gains her wits and cuts her a few times, but Anri quickly gains the upper hand, posing her sword against Haruna's throat again. Anri makes a small cut on Haruna's neck, having her Saika take control of her to force her to make the other children stop. The moment she does, all the people Shizuo was fighting back in the park return to their normal selves.
Once Haruna had fallen unconscious, Takashi called out to Anri. He is bewildered and tries to gather what happened when he suddenly sees the Headless Rider standing behind Anri. He tries to get Anri to run away from Haruna with him, but Anri refuses. Takashi questions why he would turn her down when he had done her a favor helping her with the bullies, but Anri reveals she had returned the favor by saving him from the Black Rider a couple days back. She then mentions how nice the Black Rider is and that it does not make sense why Takashi was running away from her in the first place, questioning what he had been doing. Takashi then finally notices the sword in Anri's hand and flees in terror. After he had run away, Anri turns to Celty, expecting her to be angry with her only to see the Headless Rider pick up Haruna and placing her on her motorcycle. Seeing Anri's confusion, Celty explains she had been watching the exchange long enough to generally understand her situation.
Later in the chatroom, Anri logs on as the user Saika and apologizes for the troubles they had caused. She logs off after saying she probably will not return to the chat, but Setton logs on, inviting her to chat if she wants to.
The following day, rumors began to spread regarding the incident which had been given the nickname "Night of the Ripper." The public assumed this night to be different from the other slashings because many of the victims were young men wearing yellow bandannas. To Kyouhei's concern, however, is that the Dollars were rumored as a potential culprit behind the attack. He notices there are even more Yellow Scarves members out on the street and more hostile than before because of the incident.
Elsewhere, Shizuo and Tom go to collect money from Takashi, who is hiding away in his house. When Takashi hears the doorbell go off, he becomes frightened, unsure who had come for him. He picks up a fire extinguisher, preparing to use it as self-defense against whoever tried to break in. Shizuo breaks off the door knob and enters with ease, and Takashi, not seeing who it was, swings the fire extinguisher at Shizuo's face. Takashi ends up being flown across the room from Shizuo's punch and knocking him out.
Meanwhile, Mikado and Masaomi go visit Anri in the hospital to check up on her. Masaomi leaves early, receiving a call, and leaves Mikado and Anri to talk by themselves. Mikado, seeing this as his chance, asks Anri if there is anyone she liked, but Anri only mentions the people she looks up to: the Headless Rider and Shizuo Heiwajima. Mikado is flustered, trying to come up with a response to deter Anri away from the abnormal people and that world, saying they were out of what they experience in their normal lives. Anri replies simply that to her a truly abnormal world would be one where the same routine happens everyday without change and that she was now back to her normal life. After Mikado goes home, Anri reflects on the person responsible for orchestrating the events.
Over in Shinjuku, Izaya explains to Namie everything he had set up, such as telling Takashi where Anri lived so he could coincidentally arrive while Haruna was there, although admits he did not expect the true Saika wielder to make an appearance after Takashi had stole money from him. Nonetheless, the extra factor does not bother Izaya and is satisfied this would create more conflict between the three main factions in Tokyo: the Dollars, the Yellow Scarves, and the Saika Army.
In an abandoned warehouse, Masaomi Kida is holding a Yellow Scarves meeting in their hideout. As the shogun, Masaomi proclaims he will find and destroy whoever the slasher is for destroying his peaceful life after the slasher - to his knowledge - had hurt Anri. Masaomi announces they will be targeting the slasher and if the Dollars are involved, they would go after that group as well from the inside.