After crossing paths with Muzan, Yoriichi immediately senses his violent intent, comparing it to "magma erupting from a volcano." He reveals that Muzan was the first to instigate the confrontation after stating his disinterest in swordsmen that utilize breathing techniques. As he lunged towards Yoriichi, it became apparent that Muzan possessed great skill and speed. It was the first time that Yoriichi had sensed a presence of danger.
Using his Transparent World ability, Yoriichi was able to distinct Muzan's anatomical difference that set him apart, he had a total of seven hearts and five brains. However despite his immense skill, Yoriichi was still able to successfully complete his sword forms and demobilizes Muzan. (As the flashback progresses at this point, Tanjiro's conscious within Sumiyoshi's body realizes that Yoriichi had mastered the unknown 13th form and eagerly listens.)
Despite trying to regenerate his body, Muzan becomes stunned and is forced to hold his head in place to keep it from falling off. Yoriichi realizes that the immense amount of damage was caused by the bright red blade that the wielded. He question's what Muzan's value of life was, hoping to seek an answer but only received a glare of anger.
Yoriichi's attention is then drawn to the young Demon girl —revealed to be Tamayo— travelling with Muzan. He immediately notices her eyes staring directly at Muzan, not with a look of concern, but with a look of hope as he sat there with a severed head.
Before he began dealing with her, Yoriichi began to approach Muzan in order to finish him off. However as he walked towards him, the sounds of teeth grinding grew louder in volume until Muzan's body burst into 1,800 pieces. Caught off guard by the sudden escape, Yoriichi reveals that he was only able to obliterate 1,500 of the pieces, allowing some of him to escape.
Tamayo collapses on the floor and begins to weep while tearing out her hair, she angrily curses while wishing that Muzan had died. Yoriichi approaches her and calms her down before she eagerly informs him about Muzan, how he was the progenitor of Demons and would most likely never appear before Yoriichi again. He asks for her compliance in defeating Muzan, to which she agrees, and allows her to escape.
After his interactions with the Demons, Yoriichi is approached by other Demon Slayers whom inform him that his brother, Kokushibo, had become a Demon. Once they return to the estate, he is angrily confronted by the swordsman for failing to defeat Muzan and for allowing a Demon to escape. As a result, he was banished from the Demon Slayer Corps and was demanded to commit seppuku, but the newly assigned Oyakata would not allow it to happen.
After telling Sumiyoshi the end of his flashback, Yoriichi reveals his thoughts of being born with the gift to defeat Muzan but his failure to do so, and that because of his mistake, many more people would begin to perish at the hands of Muzan.
Distraught by his inability to comfort his friend, Sumiyoshi sits beside Yoriichi in silence until baby Sumire slowly moves towards the swordsman. She tugs softly on his clothes and says "Hug!" Caught off guard by the child's sudden demand, Sumiyoshi asks if his guest would mind carrying her to make her happy. Without a response, Yoriichi picks up the little girl and lifts her up into the air.
The little girl squeals with delight and happiness causing Yoriichi to emotionally break down as his eyes fill with tears. At that moment, Suyako returns from gathering chestnuts in the forest and discovers Yoriichi weeping as he hugs the little girl.
Suyako comes forth to comfort Yoriichi saying she'll treat him to a warm meal. the chapter concludes with Tanjiro (inside of Sumiyoshi's body) wishing Yoriichi's heart to find any form of peace.