After Tanjiro Kamado and Inosuke Hashibira have successfully defeated Enmu and escaped the out of controlled Demon train with Nezuko Kamado, Zenitsu Agatsuma and Kyojuro, the Flame 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 the Total Concentration: Constant (
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 a strange human figure with glowing eyes to which Tanjiro looks at and sees the specific kanji in his irises revealing it to be Upper Rank Three, deeply horrifying him. He then becomes incredibly perplexed to why an Upper Rank would be here of all places. Before adding on to his confusion, the Upper Rank almost instantaneously dashes towards Tanjiro swinging his fist to crush his skull. Kyojuro quickly unleashes the Flame Breathing, Second Form: Rising Scorching Sun, which cuts the Demon's arm in half and causes him to leap backwards, away from the Demon Slayers.
The Demon immediately reassembles his arm and compliments Kyojuro's Nichirin Blade while licking the blood off his own arm, whom the latter 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 despises him 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 practically complete opposite moral values, and the Upper Rank then asks if Kyojuro would like to become a Demon himself.
Kyojuro immediately 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 right now.
Kyojuro says that growing old and dying is the very beauty of being human, going on to explain that people are only lovable 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 ever become a Demon, no matter the explanation.
Disappointed, the Upper Rank begins to use his Technique Development, Destructive Death: Compass Needle, Akaza says that he will just kill Kyojuro if he won't become a Demon. He quickly dashes towards Kyojuro and the Hashira returns his advance, readying the 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 only seeing sparks in the process. 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.
Being launched into the air and upside-down, Akaza unleashes his Destructive Death: Air Type, which Kyojuro counters with the 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.
Inosuke arrives and 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 bat an eye at "the weaklings" and to focus on who he's fighting and Kyojuro, angered, uses his blade to launch Akaza into a nearby forest then proceeds to run inside to finish him off. To his surprise, Akaza ambushes him from below, complimenting his exceptional movements. Kyojuro immediately dodges his attack and cuts of his arm, but Akaza retaliates by using his legs to launch him outside the forest.
Tanjiro and Inosuke call out to him, worriedly as Kyojuro, slightly enervated stumbles to get up. Akaza regenerates his lost arm and tells him to continue fighting and says that he has no other option. Which Kyojuro unhesitantly refuses and reiterates his hatred towards him. Kyojuro then dashes toward Akaza and performs the Flame Breathing, Third Form: Blazing Universe, which creates a shallow wound near Akaza's shoulder which he heals instantly while expressing his amazement over Kyojuro's beautiful attack. Akaza unleashes his Destructive Death: Air Type technique which Kyojuro barely blocks the shockwave and flies backward, then dashes towards him and continues his assault.
Inosuke looks on in amazement and horror, remarking that even if he wanted to help we would only be a hindrance, and that only death awaits if he even tries to approach them, as both seem like they're in a completely separate dimension. As both the Hashira and Upper Rank continue to exchange blows, Akaza queried whether Kyojuro is certain in continuing to fight him, as it might spell his end. Kyojuro ignores his banter and continues to attack but gets countered by Akaza's fist which creates a shallow cut on the left side of his forehead. Kyojuro backs up momentarily then proceeds to use the Flame Breathing, First Form: Unknowing Fire which severs both of Akaza's arms which regenerates almost instantly. He then calmly remarks that it would be a pity to kill Kyojuro right then as he hasn't reached his full potential yet. Akaza's suddenly dashes forwards and punches Kyojuro's guts, which causes it to bleed. Kyojuro coughs out saliva and vomit but remains steadfast and performs the Flame Breathing, Second Form: Rising Scorching Sun. Akaza dodges his technique and attempts to attack Kyojuro while he barely manages to parry all his blows with his katana. While preparing to attack Kyojuro with a stronger attack, Akaza continues to convince him to become a demon, saying that in one or two years his strength and power would be able to grow exponentially.
Akaza attempts to attack Kyojuro with a strength-enhanced strike, to which Kyojuro barely blocks his attack. However, Akaza's strength was able to overwhelm Kyojuro and crush his eye with his fists. Kyojuro stumbles back but continues to grit his teeth in the onslaught he is facing and uses both his Flame Breathing, Third Form: Blazing Universe and the Flame Breathing, Fourth Form: Blooming Flame Undulation. Akaza dodges both attacks and takes his stance while Kyojuro does the same, both then proceed to unleash different techniques, the former using his Destructive Death: Disorder attack while the latter using the Flame Breathing, Fifth Form: Flame Tiger. Both close in on each other and Kyojuro manages to cut of Akaza's arm and cause a shallow cut on his chest. The residue of their attacks clear, and Akaza tells Kyojuro, who's now bloodied and beaten, having lost the functionality of 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 him forcefully switching from the Water Breathing to the Hinokami Kagura, which he used to defeat Enmu. Kyojuro turns back to face Akaza, still 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.
Kyojuro commands himself to set his heart ablaze and burn beyond his limits. With that, he proudly proclaims his identity as the Flame Hashira as he readies his family's most powerful Breathing technique, the Flame Breathing, Esoteric Art: Ninth Form, Rengoku. Akaza also readies his Destructive Death: Annihilation Type, and the dust this time disperses to reveal that Akaza has stuck his arm through Kyojuro's solar plexus. Akaza screams that he's going 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.