Muichiro sees Kotetsu being attacked by a Demon. He contemplates saving him but suddenly remembers Tanjiro Kamado's words and beheads the Demon, telling Kotetsu to escape. When Kotetsu intensely thanks him and apologizes for his original hatred towards him, Muichiro's memories spur as he remembers recovering under the care of the Ubuyashiki family.
Kotetsu leads Muichiro to where Kozo Kanamori is. As Kozo starts making for the shed to retrieve the new blade, Muichiro pulls him back as he senses the presence of Upper Rank 5, Gyokko. Muichiro attacks him, but Gyokko moves to different pots and avoids his slashes. He summons more fish Demons that send an array of needles towards Kozo and Kotetsu, but Muichiro shields them and tells them to move out of the way. Gyokko reveals the needles are poisonous and berates Muichiro, resulting in another sudden memory recurring to him. He tries to attack Gyokko, but gets trapped in his Blood Demon Art, Water Prison Pot, his attempt to break out using Mist Breathing, First Form: Low Clouds, Distant Haze unsuccessful. As he thinks about his approaching demise, he suddenly hears his father speaking from Tanjiro's body. Kotetsu swiftly arrives to provide him with a breath of air and he is able to break out using Mist Breathing, Second Form: Eight-Layered Mist, remembering it was his father who said the same thing as Tanjiro had earlier.
After escaping from the water prison, Muichiro's memories of his past all come back. The anger over his brother's death manifests into his Demon Slayer Mark, his speed and abilities dramatically increasing as he goes into the shed and attacks Gyokko, who comments on his relaxed face. Gyokko retaliates with his Blood Demon Art, Octopus Vase Hell, and Muichiro's blade breaks trying to cut it, prompting Kozo to quickly hand him his new one. Muichiro slices through the Demon's attack and thanks Kozo, in which he replies that he only followed the instructions of the late Tetsuido. At the mention of his name, Muichiro remembers his old swordsmith's worry for him and internally apologizes, remarking that he was okay now and uses Mist Breathing, Fifth Form: Sea of Clouds and Haze, landing a blow on Gyokko's neck.
Muichiro and Gyokko start insulting each other, with the latter growing furious after the former insults his pot. He uses another Blood Demon Art, Ten Thousand Gliding Slime-Fish, but Muichiro easily counters it with Mist Breathing, Sixth Form: Lunar Dispersing Mist and Mist Breathing, Third Form: Scattering Mist Splash, slicing the head of the demon's molt as he escapes to a tree. When Gyokko tries to show off his new form, Muichiro's indifferent attitude towards it enrages him again and he attacks, which Muichiro dodges and smiles, commenting on the uselessness of his offense.
Another scuffle between the two ensues, with Gyokko using his Blood Demon Art, Killer Fish Scales and Muichiro responding with the form he personally developed: Mist Breathing, Seventh Form: Obscuring Clouds.When Gyokko tries to find him in a sea of mist, Muichiro effortlessly decapitates him.
After the battle, Muichiro starts foaming at the mouth due to poison and falls unconscious after verifying Kotetsu is alive, his family congratulating him on a job well done in his consciousness.