"Perpetual Shifts and Changes" is the seventeenth episode of the Durarara!! anime and the eighteenth episode overall.
The episode opens with Anri holding Saika while Haruna stares and Nasujima huddles nearby, terrified. In South Ikebukuro Park, Shizuo is fighting the Saika Army as Celty watches in awe. At the moment that Anri pulls Saika from her arm in another part of the city, all of the people in the park stop fighting and turn in her direction. As Celty leaves the park to investigate, she tosses Shizuo a pair of gloves made from her shadows that are strong enough to withstand the blades he is fighting.
Anri disarms Haruna and asks her to order her army to stop fighting. She pierces Haruna with the tip of Saika, and Saika's words of love resound through Haruna's mind. In a flashback, Anri's happy dreams of times with her parents are replaced by the reality of her father abusing her and her mother, and she sees the day five years prior when her mother, Saika's previous host, killed her husband and herself. 10 year-old Anri picked up the katana and immediately bonded with Saika.
In the park, the cuts Shizuo receives from Saika's army have no effect on him. The slashers wait for him to open up to them, but his lack of fear keeps their thoughts from entering his mind. To himself, Shizuo admits to feeling fear, but he is afraid of himself losing control. He decides to give this fight his all, rejecting the notion of going down in front of people who love him.
Haruna asks how Anri can bear Saika's voice. Anri responds that because of her inability to love, due to closing off her heart when her father abused her, she leeches off of others to compensate for the qualities she lacks, and this allows her to bear Saika's words of love. Haruna picks up her knife and cuts Anri several times, but the cuts do not effect her. Anri takes the injuries as her consequence for what she is about to do to Haruna in order to protect the peace in her life.
As Anri takes over Haruna's mind, Haruna and all of the people in the park awaken from their possessed state. Shizuo manages to stop himself in action is delighted to discover that he didn't kill anyone in the fight and was finally able to control his body.
Celty catches up to Anri, and Nasujima tries to take Anri away. Anri explains that she has already repaid him for the times he looked out for her at school by protecting him when Celty chased him. Celty remembers the night six months earlier when she chased a man carrying a case and was attacked by the Slasher. She identifies the man she'd been chasing as Nasujima. Anri promises that she won't hurt him, because she isn't like Haruna, and Nasujima runs away.
Kyouhei's gang are riding in their van elsewhere in the city. Saburo is seething because the door Walker found to replace the one Shizuo tore off has a large image of an anime girl on it. Kyouhei warns them that they can't get into trouble in their van anymore because of how much the door makes it stand out.
Mikado checks on Anri at the hospital and she tells him about the night she had, in which she encountered the Slasher twice and was twice saved by the Headless Rider. Mikado forces himself to keep from telling Anri that Celty is a woman and that he knows her. Anri thinks over the night's events, ending when Celty took care of Haruna and told Anri to stop doubting herself but use her power for good. When Celty took responsibility for Haruna and the Saikas she had created, Anri took Haruna's memories and discovered the name of the person behind Haruna's actions, determined to remember the name Izaya Orihara and protect her friends from him.
In Izaya's apartment in Shinjuku, he reveals to Namie that he had sent Nasujima to Anri's apartment. Nasujima had owed money to the Awakusu, so he tried to extort money from Haruna Niekawa's parents. The Awakusu sent Nasujima to Izaya to get information on the Niekawa family, and Izaya left an open briefcase of cash on his desk as bait. Nasujima stole the money, and Izaya sent Celty to retrieve it. Izaya had plans to threaten Nasujima to get to Haruna, who he knew to be a variation of Saika, but he hadn't anticipated Anri's arrival. He confesses that everything still worked out as he wanted it to.
After running from Anri the night of the battle between the Saika Army and Shizuo, Nasujima, desperate to pay back his debt, tried to mug the first person he saw and was beaten up by Shizuo.
In Ikebukuro, Kyouhei's gang are watching a large number of police officers walking the city. Some are apprehending members of the Yellow Scarves. They talk about news and rumors surrounding a huge fight in South Ikebukuro Park. That night, they had picked up a beaten and disheveled Nasujima and questioned him, but as he was unable to remember a thing, they left him tied on the side of the road. The police and some people online believe that the events of that night were a fight between color gangs. At once, all four receive emails from the admin of the Dollars seeking information on the Slasher. Outside the hospital, Mikado puts his phone away.
In Shinjuku, Namie asks Izaya what he is planning. He seems satisfied that tensions are rising between the Dollars and the Yellow Scarves with members of Saika's Army in both gangs. Namie questions the strength of the Yellow Scarves, who were started by a middle-schooler, but Izaya assures her that anyone who can lead a force the size of the Yellow Scarves is a 'serious threat.'
In a warehouse in another part of the city, Masaomi is sitting on a couch with a yellow bandanna in his hand. The warehouse in full of members of the Yellow Scarves who beg him to return. Horada demands they call him 'chief' and show him respect. Masaomi waves it off, telling them he's no longer their leader and that he enjoyed the peace that being a high schooler brings him. That peace was disrupted when Anri was attacked by the Slasher. He rises, putting on the yellow scarf, and tells the warehouse full of people that they are going after the Slashers and, if they have to, the Dollars as well.
- To the time Celty was attacked by the Slasher while chasing a man carrying a case
- To the time Shinra gave Celty a job from the Awakusu to retrieve something before delivering it
- Anri Sonohara
- Sayaka Sonohara
- Haruna Niekawa
- Takashi Nasujima
- Celty Sturluson
- Shizuo Heiwajima
- Saburo Togusa
- Kyouhei Kadota
- Walker Yumasaki
- Erika Karisawa
- Mikado Ryuugamine
- Namie Yagiri
- Izaya Orihara
- Shuuji Niekawa
- Masaomi Kida
- Walker and Erika reference the series Kino's Journey
- As in episode 11, Walker's ringtone is Ono Daisuke saying "Bocchan, meeru de gozaimasu" which translates as "Young master, you've got mail." Ono Daisuke voices Sebastian Michaelis in Black Butler (also known as Kuroshitsuji) and is also the voice actor for Shizuo Heiwajima.
- Erika's ringtone is the voice of character Ai Enma from the anime Hell Girl (Jigoku Shoujo) saying "Ippen shinde miru?" which roughly translates to "Do you want to trying dying this once?"
Shizuo: "There's no way I'm going down in front of the people who love me."
Anri: "Tell me, Mikado. What do you think a truly unusual life would be like? Something really unique. It'd be the kind of life where, every single day, nothing new ever happened, nothing changed. That kind of a life is truly unusual."
Celty (to Anri): "If you still have any doubts, I suggest you stop questioning yourself, and instead use that power you have to better this town."