Seeing Tanjiro try to protect Nezuko even as he collapses triggers memories of Rui’s own past. Once a sickly boy, he became a demon after meeting Muzan Kibutsuji, who gave him a share of his blood. But after he’d turned into a demon, his own parents try to kill him. Ever since then, he’s been searching for a family. Seeking a bond that seemed so unattainable…
On a winter day, a young boy watches as children play in the snowy streets. As he approaches them, he falls to the ground and struggles to breathe. He turns over and looks to the sky, shortly as a woman tends to the boy; her son Rui. His mother scolds him as he knows he isn’t meant to be outside.
Rui recalls his past; from his birth, he had always been frail. He couldn’t even run as walking was challenging for him, until Muzan appeared before him one night. The Demon Progenitor “pitied” him and offered a way to save him. His parents soon learned what became of their son. Despite now possessing a physically strong body, he could no longer be in sunlight and had to devour humans to sustain himself. He remembers when his father and mother first discovered when he ate someone; his father in utter disbelief and shock, while his mother cried in devastation.
Rui thinks back to a story he heard growing up, about a man who died attempting to save his drowning son, and how he was so moved by an act of deep parental love and such a close bond. To him, that man had fulfilled his role as a parent; subsequently, he never understood why his parents tried to kill him. One night, as he slept, his father raised a knife to his son, while his mother did nothing to prevent it and sobbed next to the bedroom.
Rui awoke and killed them in retaliation. Afterwards, he sat on the porch of his house as he reasoned that his parents were imposters, not believing they had a bond. As he sat there, he hears his mother’s last words, an apology for not giving him a strong body. When she died, he suddenly remembers his father’s last spoken words to him, “It’s all right, Rui. We’re going to die with you”.
Rui was so enraged at his father’s attempt to kill him, he ignored his last message; he was willing to die with him to atone for his murder of another person. Realizing then that they did have a genuine bond, he laments that he had severed it himself. Muzan then came to him and pushed the contrary, telling the young demon it was the fault of his parents for their inability to accept him. Devastated by what he had done, Rui forced himself to believe his words, unable to bear the weight of his act.
As he dies, Rui’s headless body walks to Tanjiro; even as he understood that he was solely to blame, Rui missed his parents every single day since. Even after creating a fake family, the feeling of emptiness never left him. And because he was the most powerful demon in his family, no one could protect him. As Giyu watches, Rui’s disintegrating body makes its way to the injured Demon Slayer; Rui states that as he got more powerful, he remembered even less about being human, and as he did, he lost sight of what he wanted; to seek a bond he could never claim.
Rui falls just of Tanjiro as he continues to fade, and he tries in vain to reach out to him. Tanjiro looks at the body and senses the scent of unbearable grief. He gently touches the Lower Rank, the demon experiencing the feeling of warm, gentle sunlight. Rui remembers that he wanted to apologize to his parents, hoping they could forgive him. Nonetheless, he knows that killing scores of people will bring him to Hell anyway, denying him the chance at forgiveness from his parents.
In the afterlife, a voice reassures Rui, “We’ll be going together even if it’s to Hell”; his father. Shocked to see him, his mother then appears. She comforts her son, “No matter where you go, we’ll be with you”. Rui’s appearance changes back to when he was human as he hugs his parents. He tearfully apologizes for everything he had done, as the fires of Hell consume their souls; the last physical remnants of Lower Rank Five now gone.
As Tanjiro watches Rui’s body fade away completely, he is shocked to see Giyu stepping on his clothes. The Hashira scolds him from showing sympathy to a demon, telling him it is irrelevant it looked like a child since it is a hideous monster that has lived for decades. Tanjiro responds that to avenge those killed by demons and prevent more victims, he will kill without mercy, but to demons who despaired and regretted their actions, he would never trample on them because they were human too, begging Giyu to step off Rui's clothes. He claims that demons are not hideous monsters but tragic and hopeless creatures. As Giyu looks at him and sees Nezuko as well, he suddenly remembers the two from their encounter years ago.
At that moment, butterflies surround Giyu, though he quickly understands the situation and lowers his stance. From the trees, Shinobu speeds in and attempt to kill Nezuko; Giyu quickly blocks her blade and she becomes confused. She asks why he interrupt her attack after stating himself that demons and humans could never get along, further stating that is why he is disliked.
Elsewhere, a transformed Demon Slayer is given a dosage of medication and the poison quickly disappears. The person who administered it, a man wearing clothes titled “Kakushi”, exclaims his amazement at its effectiveness, as it is Shinobu’s own making, while his female partner adds that Shinobu had it determined after one look of the situation.
She calls the Hashira amazing but also claims that Zenitsu is too, as he is brought down the suspended house; he had killed the demon responsible and its death had slowed the poison’s circulation slightly. The Kakushi applies a sign to his patient explaining his finished treatment and gladly states that all those affected can recover, while the female Kakushi comments that it is wise to pack up and leave the scene quickly. As Tanjiro looks at Shinobu’s unusual blade, she asks Giyu to move again. He responds that he isn’t disliked by people, to Tanjiro and Shinobu’s surprise.
She gives an apology and states she is unaware he has not realized it, to Giyu’s chagrin and Tanjiro’s further shock at the situation. She calls out to Tanjiro and tries to tell him he is protecting a demon; Tanjiro states he is aware but adds that the demon is his little sister. Shinobu’s way of showing pity is offering a gentle toxin to kill her without pain, worrying him. Giyu asks if he can move, then instructs him to take his sister and run. Without hesitating, Tanjiro thanks him, takes Nezuko and flees. Shinobu claims that is against the Corps rules but Giyu ignores her.
With Nezuko in his arms, Tanjiro runs far away through the forest, taking his box upon spotting it. Despite being in intense physical pain, he forces himself to keep running. As Nezuko awakens, he wonders if he must quit the Demon Slayer Corps, as they would surely not allow a swordsman to travel with a demon; unbeknownst to Tanjiro, someone is pursuing him through the trees.
Shinobu and Giyu clash swords, with her in disbelief at a Hashira protecting a demon. When Giyu doesn’t say a word, she decides to leave him to avoid stalling time, before leaping into the trees to hunt down Nezuko. Shinobu daintily weaves through the forest as Giyu chases her from the ground, sarcastically ask if that is how he will catch up to her. She leaps high into the sky and states she is willing to let him try and stop her; at this, Giyu leaps towards Shinobu with great height, much to her surprise.
As Tanjiro continues to make his way down Mount Natagumo, a girl descends from behind him and lands on his back, causing him to fall down and lose hold of his sister. He sees Nezuko but also sees another Demon Slayer has appeared as well; before she can behead Nezuko, Tanjiro grabs the girl’s cloak to make her lose balance. He tells Nezuko to run but is knocked unconscious by the Demon Slayer. She gets up and attempts to behead the demon but Nezuko shrinks down to avoid the swing. Confused at her attempt to escape her, the girl chases her down but repeatedly misses her slashes as Nezuko evades them. She notes that she isn’t attacking back but decides not to think about it, given the one task to kill demons.
Meanwhile, Shinobu attempts to talk to Giyu while being restrained by him; she states what she did is a clear and justified attempt to kill a demon and asks what were his intentions when he caused her attack to miss. When Giyu doesn’t say anything again, Shinobu firmly asks again and gives him a warning. He states it was because of something that happened two years ago but Shinobu doesn’t understand it, asking him not to ramble about something in the past. She questions if he is being spiteful because she had pointed out he is disliked by people, annoying Giyu again.
Shinobu attempts to escape his hold by deploying a blade in her shoe, but before she can bend back to strike him, a Kasugai Crow announces a message, stopping her just short of hitting his face. The crow gives a message from headquarters; Tanjiro and Nezuko are to be taken into custody and brought back. Another crow announces a description of the two; Tanjiro is clad in a checkered haori, with a scar on his forehead, while Nezuko is a demon wearing a bamboo muzzle. The Demon Slayer chasing the two hears this and asks Nezuko on her identity. Giyu and Shinobu sheath their swords and leave Mount Natagumo, while Tanjiro is found by Kakushi going by his description from the crow.
Another pair of Kakushi find Inosuke still tied to the tree, but are wary after seeing his boar mask. They eventually discover he is indeed a Demon Slayer and take him down, while shocked at the extent of his injuries; as he is taken away, Inosuke mutters to himself about his inability to kill even a single demon and begins to doubt himself.
Elsewhere, Zenitsu, with Chuntaro as company, has finished his treatment and put alongside the other treated Demon Slayers. He wonders about the Kakushi and comments on their speed and efficiency. He sees the girl giving orders and recognizes her as a survivor in Final Selection. The girl tells the Kakushi to bring the wounded to her place, the so-called Butterfly Mansion, while she hunts demons around the forest to protect them.
Zenitsu then remember who the Kakushi are; they serve as clean up crews after battles with demons, and are composed of people without skills in swordsmanship. Dawn breaks over Mount Natagumo as everyone leaves the forest; Tanjiro now asleep on a Kakushi’s back.
Tanjiro, still asleep, lies on a stone garden when a Kakushi commands him to wake up. He jolts awake and finds himself before several swordsman; the Kakushi telling him he is in the presence of the Hashiras.
Characters in Order of Appearance