Tanjiro and Giyu are running in the castle, trying to find Muzan Kibutsuji and Tamayo. They have just heard about the death of Shinobu Kocho at the hands of Upper Rank 2, Doma. Suddenly, a tremor appears, shaking both the Demon Slayers. Tanjiro recognizes a familiar smell and the roof above them breaks, revealing Upper Rank 3, Akaza. Akaza is surprised that a 'weakling' like Tanjiro is still alive and shouts his name, with Tanjiro screaming his name in return. The fight between the three begins, a way for Tanjiro to avenge the death of the Flames Hashira, Kyojuro Rengoku, a man Tanjiro admired a lot, who died at the hands of the Demon.
As Akaza advances, Tanjiro dodges his punch and uses Hinokami Kagura: Fire Wheel on Akaza and, with considerable effort, severs the demon's arm. As Tanjiro tries to recover, Akaza attempts to behead Tanjiro with a back-fist punch which Tanjiro dodges and counters with Hinokami Kagura: Fake Rainbow managing to create a long cut on Akaza's face. Looking at this fight, Giyu is quite amazed and surprised by Tanjiro's sudden and quick improvement, comparing his abilities to that of a Hashira. Akaza remembers Kyojuro's words about Tanjiro not being weak and accepts it as a fact, deciding to acknowledge him by activating his Technique Development and indicating the start of the real battle.
Water and Fire
Giyu enters the fight, immediately using Water Breathing: Third Form - Flowing Dance. Akaza is instantly impressed as it had been 50 years since he ran into someone as good as Giyu. He uses Destructive Death: Chaotic Type, a flurry of punches that produce giant shockwaves, which Giyu negates using his own creation, Water Breathing: Eleventh Form - Dead Calm. Akaza remarks that he had never seen this technique used by the previous Water Hashira he last fought. While Tanjiro tries attacking from behind using Hinokami Kagura: Raging Sun, Akaza appears from behind and attempts a backhand chop but Giyu cuts his attacking hand with Water Breathing: Second Form - Water Wheel. As Giyu attacks the demon, Tanjiro tries to trip Akaza by using Hinokami Kagura: Flame Dance on his legs, however, he is countered by Destructive Death - Leg Type: Crown Splitter. Even though Tanjiro is able to block the technique, he is hit hard enough to spit blood. Akaza continues to battle Giyu and is impressed by the Hashira's swordsmanship, asking for his name. Giyu refuses and Akaza replies that he would ask again and again and kicks him with his Destructive Death - Leg Type: Explosive Furry, launching him through various walls to another side of the castle.
The Strong Should Aid and Protect the weak
Akaza figures out the Water Hashira's name after Tanjiro shouts for him when he is blasted away by Akaza's previous attack. Tanjiro and Akaza attack each other with Hinokami Kagura: Burning Bones, Summer Sun and Destructive Death: Eight-Layered Demon Core, respectively.
Akaza praises Tanjiro's improvement and claims that Kyojuro did an amazing job, pleased with his death. This enrages Tanjiro and prompts Akaza to then gave his philosophy of how he hates the weak and wish they got eliminated. Tanjiro retorts by saying that even Akaza was once a child and that someone had helped him. He continues by stating the strong should aid and protect the weak and he wouldn't let Akaza do as he pleased any further.
Tanjiro's words shakes Akaza as he feels restless and memories of Keizo appear, saying to him the same words. Akaza brushes it off and realizes it was but a hallucination. Angered, he claims Tanjiro to be unpleasant and uses Destructive Death: Crushing Type - Ten Thousand Leaves Flashing Willow and becomes much more faster as Tanjiro attempts to dodge. The demon surprises Tanjiro and kicks him with his Destructive Death: Leg Type - Flying Planet Thousand Wheels, making him cough up more blood. He compares Akaza's precision to that of a magnetic compass and begins to think and deduce using the demon's moves and words.
Tanjiro dodges another kick by Akaza and attacks him with Hinokami Kagura: Solar Heat Haze, Akaza managing to avoid the technique. However, a considerable portion of his neck is sliced and he expresses amazement, asking Tanjiro how he swung his blade. Tanjiro attempts to use Hinokami Kagura: Dance, but the sword is clasped in Akaza's hand and he attempts to break it.
The Mark Appears
Tanjro uses headbutts and kicks, trying to make Akaza loosen his grip despite the demon not budging. Luckily, his hands got cut by Giyu, angry about the deadly kick Akaza gave him previously. Giyu's senses awaken and a mark appears on his face. He easily dodges a shockwave punch and gives a quick slice of his own. Akaza dodges, but the attack slicks a small portion of his neck. The demon acknowledges the increase in speed as Giyu surprises him with a sudden Water Breathing: Fourth Form - Striking Tide, vertically slicing Akaza's right arm.
Tanjiro notices that Akaza has matched Giyu's speed and wondering how Akaza's attacks are so accurate. He suddenly remembers words from Akaza and Inosuke Hashibira, the latter explaining how he is able to sense killing intent and how he cannot detect people behind him if they have no killing intent.
Meanwhile, Giyu and Akaza seem to be evenly matched. Tanjiro attempts to use the Hinokami Kagura: Dance again, but was stopped by Akaza who tries to use a back-fist strike which is dodged by Tanjiro. Using this opportunity, Giyu attacks the demon. Tanjiro gets a grasp about the truth behind Akaza's techniques.
The Transparent World
As the fight between Giyu and Akaza goes on, Tanjiro remembers the time his father, Tanjuro Kamado, explains to him about correct breaths and movements and about being able to see the Transparent World, which one can achieve if the mind becomes invisible. He demonstrates by killing a bear while showing no killing intent. Tanjiro manages to achieve this state for a short moment in time when Akaza attacks him, closing off other senses and focusing on dodging.
Akaza and Giyu are battling fiercely, trading blows to each other. The Water Hashira has used up all his forms during the fight and Akaza proclaims that it will be the end of the Hashira, amazed at how long he fought for. As Giyu is about to attack the demon's fist, Akaza breaks Giyu's sword by punching it from the side and attempts to punch through him, but his arm is severed by Tanjiro who had managed to enter the state. The demon notes the changes and tries to kill the young slayer. Giyu takes this moment to attack him, but Akaza uses his strongest technique Technique Development: Final Form - Blue Silver Chaotic Afterglow. Giyu tries to reduce it with Water Breathing: Eleventh Form - Dead Calm, but the attacks were too random to completely cancel out and he gets hit by many shockwaves despite managing to dodge several blows.
Meanwhile, Tanjiro is able to see everything move slowly in the Transparent World. Akaza is unable to notice that Tanjiro was behind him and Giyu hopes he goes for the neck that instant. However, Tanjiro calls the demon out and Akaza turns to strike, but Tanjiro dodges. Akaza could not detect his battle spirit via the compass and Tanjiro manages to decapitate Akaza with Hinokami Kagura: Setting Sun Transformation.
“Hakuji-San, stop already!”
Akaza was bewildered, wondering how Tanjiro did not have a battle spirit, comparing this feeling to fighting against a plant. He acknowleges that Tanjiro surpassed his speed. He tries to attach his head back, but Giyu throws his broken sword, piercing the head, separating it from the body. His head disintegrates but his body continues to fight, using his Technique Development to set up a compass while his head portion regenerates.
Tanjiro dodges his chops but is thrown aside by the demon's kick. Tanjiro tried to get up and fight, but the stress and exhaustion of the fight caused him to pass out. Giyu protects Tanjiro with his Water Breathing: Fourth Form - Striking Tide, slicing the headless body in various regions, but the demon regenerates faster. The two began to trade blows and although Giyu is badly hurt, he still decides to protect Tanjiro, giving Akaza vague memories of Keizo. As he advances, he was stopped by the teary-eyed spirit of his human self's deceased fiance, Koyuki, telling him to stop.
Giyu continues to stand up, despite being burdened with the desire and goal to not let his friends or family die in front of him (like his sister and Sabito). Akaza was held by Koyuki's hand as his head continues to regenerate.
Akaza begins to remember why he wanted to become strong and thinks back to the time he was the "Demon Child" Hakuji, from his father's death to meeting Keizo and Koyuki. He remembers trying to find a purpose and reason in life, finding someone to protect. His memories of his past suddenly come rushing back.
During this, Tanjiro manages to wake up and Akaza's head has halfway recovered. The demon attempts to kill Giyu with his Destructive Death: Annihilation Type and Tanjiro rushes to stop him, but his sword slips and he punches Akaza, causing him to question his current deeds and begins to regret sullying his master's precious style. As he prepares his technique, Tanjiro tries to pull Giyu away but, with a smile of gratitude in his face, Akaza uses the technique on himself, mutilating him and reducing many parts of his body to bones.
Akaza admits defeat, stating that Tanjiro's technique was magnificent and that he wants to go to hell like a man. He imagines his father and Keizo, but when he rememebers Muzan asking if he wanted to get stronger, he began to regenerate. He is stopped by Koyuki, thankful about Hakuji recovering his memories and thanking him for what he had done. Finally finding peace, Akaza disintegrates, dying as Hakuji: a loyal son, a protective student, and a faithful lover.
Hakuji reunites with his father, Keizo, and Koyuki in hell. Tanjiro is glad the battle is over and wants to reach Tamayo, but he faints again. Giyu also loses consciousness and slumps onto his sword out of fatigue. Akaza's death is spread by a Kasugaigarasu and was heard by the other Upper Ranks. Kokushibo laments that he could not get any stronger and called him 'weak'. Doma begins to openly weep and mourn his 'best friend', though his actions are later proven to be a facade.
Shortly after the fight against Kanao and Inosuke against Doma, Tanjiro wakes up with Giyu cauterizing wounds and receives knowledge of the death of Upper Rank 2.