After Tanjiro Kamado and Inosuke Hashibira have successfully defeated Enmu and escaped the out of control Demon train with Nezuko Kamado, Zenitsu Agatsuma and Kyojuro, the Hashira checks on an injured Tanjiro as Inosuke goes to rescue another passenger. He applauds the young Demon Slayer for his recently-acquired ability to use Total Concentration Breathing all the time, and says that this is the first step to him becoming a Hashira.
Kyojuro then notices that Tanjiro is bleeding from the abdomen and tells him to focus his breathing in order to heal himself, which the young Demon Slayer manages with some guidance from Kyojuro. The Flame Hashira tells Tanjiro that if he can master his breathing, he'll be able to accomplish many more things and become stronger tomorrow than he was yesterday.
Just as Kyojuro starts to tell Tanjiro that he doesn't need to push himself any further because everyone's been saved, something jumps down just behind the two Demon Slayers with a great force, scattering the dust around them.
The dust clears to reveal Upper Rank 3, who immediately launches himself towards Tanjiro with the intention of punching his head in. Kyojuro quickly unleashes his Flame Breathing: Second Form - Rising Scorching Sun attack, which cuts the Demon's arm in half and causes him to leap backwards, away from the Demon Slayers.
The Demon immediately regenerates his arm and compliments Kyojuro's katana, who then asks him why he was targeting an injured person. The Upper Rank says that he just thought Tanjiro would get in the way between himself and Kyojuro, to which the latter responds by asking if they had something to talk about and saying that he hates the Demon despite it being their first time meeting. As he smiles, the Upper Rank says that he hates weak humans too and that they disgust him. Kyojuro responds that the two appear to have very different moral values, and the Demon then asks if Kyojuro would like to become a Demon himself.
Kyojuro swiftly declines, but the Upper Rank doesn't back off, saying that he knows the Hashira's strength just by looking at him and that he's getting close to supreme territory. Kyojuro says that he is the Flame Hashira, Kyojuro Rengoku, and the Demon introduces himself as Akaza. The Upper Rank then goes on to explain that, despite him being so close to it, Kyojuro will never reach supreme territory because he'll just grow old and die before it can happen. Akaza says that if he becomes a Demon, Kyojuro can keep training for hundreds of years to become even stronger than he is now.
Kyojuro says that growing old and dying is the very beauty of being human, going on to explain that people are only loveable and precious because of this aspect. What humans call strength, the Hashira says, isn't a word that they use in regards to the body, and that Tanjiro is not weak and should not be insulted by Akaza. Kyojuro looks the Upper Rank in the eyes and tells him that he will never become a Demon.
As he begins to use Technique Development: Destructive Death: Compass Needle, Akaza says that he will just kill Kyojuro if he won't become a Demon. He runs towards Kyojuro and the Hashira returns his advance, readying his Flame Breathing: First Form - Unknowing Fire attack, and Tanjiro notes that they're moving so fast that he can't follow them with his eyes. They clash, and Akaza leaps up in the air, saying that none of the Hashira he's killed so far used flames and that none of them ever agreed to his invitation to become a Demon. Akaza says that he finds them hard to understand, especially since only those who are chosen get to become Demons. The Upper Rank says he is saddened by the thought of the decline of someone wonderfully talented, and tells Kyojuro to die now, while he's young and strong.
Akaza unleashes Destructive Death: Air Type, which Kyojuro counters with Flame Breathing: Fourth Form - Blooming Flame Undulation. The Hashira notes that Akaza is attacking him from a distance, punching into the air and moving at a fraction of a blink, and realizes that he'll have to get closer to him if he's to defeat the Demon.
Kyojuro suddenly appears in front of Akaza, who compliments his reaction speed as the Hashira attacks him with his flames. The Upper Rank praises his sword skills, and wonders to Kyojuro how sad it'll be if they're lost early, to which the Hashira replies that it won't be sad because it's only natural for people to die.
Tanjiro begins to get up, wanting to help the Hashira out, but Kyojuro tells him to stay where he is, because his wound could be fatal if reopened. Akaza tells him not to take notice of "the weaklings" and to focus on who he's fighting and Kyojuro, angered, attacks the Demon with Flame Breathing: Fifth Form - Flame Tiger as Akaza unleashes his Destructive Death: Disorder.
The residue of their attacks clear, and Akaza tells Kyojuro, who's now bloodied and beaten, having lost his left eye, not to die on him just yet. Tanjiro and Inosuke panic, the latter wanting to jump in to help Kyojuro but soon realizing that he'd only be a hindrance, as the Hashira and Upper Rank seem to be in an entirely different dimension to him.
Akaza says that Kyojuro fighting with the intention of sacrificing his body is pointless, showing him that the injuries he'd inflicted on him had already completely healed and comparing this to the severe wounds that now covered Kyojuro. He explains that Kyojuro can't recover from this anymore as he is, but could regenerate himself in the blink of an eye if he was a Demon. Akaza says to an exhausted Kyojuro that humans will never be able to beat Demons.
Tanjiro tries to pick himself up to go and help Kyojuro, but is unable to generate any strength into his limbs because of the aftermath of Hinokami Kagura, which he used to defeat Enmu. Kyojuro turns back to face Akaza, smiling and saying that he'll fulfil his duties before beginning to ready another attack. Impressed, Akaza compliments his fighting spirit and technique, and laughs as he says that Kyojuro really should've chosen to become a Demon, asking the Hashira to fight with him for all eternity.
The two clash again, Flame Breathing: Ninth Form - Rengoku meeting Destructive Death: Annihilation Type, and the dust this time disperses to reveal that Akaza has stuck his arm through Kyojuro's stomach. Akaza screams that he's doing to die, and asks Kyojuro one more time to tell the Upper Rank that he'll become a Demon, saying that he's one of the strong chosen ones.
This causes Kyojuro to remember his mother, Ruka Rengoku, and a flashback shows one of their final moments together. Ruka asks a younger Kyojuro to think hard about what she's about to ask him, then presenting him with the question of whether or not he knows why he was born stronger than other people. Kyojuro ponders this for a while, then barks out that he doesn't know the answer. Ruka explains that he was given such strength to protect the weak, and that power should be used for the world and for the people, not for abusing it in order to hurt others. She says that it is the obligation of those born strong to protect those who are weak, and tells Kyojuro never to forget that.
Ruka hugs Kyojuro to her chest and, as she begins to cry, tells him that, even though she won't be alive much longer, it was a blessing for her to have been the mother of such a strong, gentle child.
Now back in the present, as he summons the last of his strength to swing his sword into Akaza's neck, Kyojuro thanks Ruka as well, saying it was an honor for him to have had her as his mother. Kyojuro lets out a shout as he drives his sword deeper in, and, as Akaza tries to hit him with a punch to the face, the Hashira grabs his wrist and stops the attack before it reaches him. Akaza is incredibly impressed by Kyojuro's strength, as he still managed to stop the Demon's attack while his fist was stuck right through his solar plexus. He then looks behind Kyojuro, and begins to panic as he realizes that the sun is about to come up. Akaza tries to free himself from Kyojuro's hold, but the Hashira holds on tightly to his wrist, not wanting to let him escape.
Tanjiro finally finds the strength to pick up his sword, and starts to run over towards the pair with the intention of dealing the finishing blow to Akaza. With a loud shout of effort, Akaza tries again to pull himself away from Kyojuro, but the grip of the Hashira, determined, remains firmly around his wrist. Kyojuro's blade moves deeper into Akaza's neck, now reaching its center. Tanjiro yells to Inosuke to move for Kyojuro's sake, and, just as the boar-headed Demon Slayer readies his Beast Breathing: First Fang - Pierce, Akaza manages to pull himself away from Kyojuro.
The Upper Rank quickly jumps backwards, both of his arms now severed, and runs into the forest beside where they'd been fighting, leaving Kyojuro to fall onto his knees, defeated.
As the Demon retreats, Tanjiro throws his sword at Akaza, screaming at him not to run away from the fight and calling him a coward. The young Demon Slayer yells that Kyojuro is better than Akaza will ever be because he fought until the end and didn't let anyone die, and that Kyojuro was the rightful winner of their fight.
Tanjiro lets out a final scream and collapses to his knees, crying, and Inosuke starts to shake. Kyojuro looks over at him and smiles, telling Tanjiro to come over to him so they can have a final chat before he dies.