Tanjiro ventures to the south-southeast where he encounters a cowardly young man named Zenitsu Agatsuma. He is a fellow survivor from Final Selection and his sparrow asks Tanjiro to help keep him in line.
Together, Tanjiro and Zenitsu venture up a mountain where they find a mysterious house. They meet Shoichi and Teruko, two small children who's brother was abducted and taken into the house. The sound of a Tsuzumi echoes through the mansion every so often, and it all belongs to a Demon.
Tanjiro and Zenitsu head into the house but are separated shortly after, tasked with protecting one of the children each. Zenitsu accidentally discovers another young demon slayer who wears a boar hide as a mask. The wild boar Inosuke charges through the house trying to slay the same demon as the others.
Inosuke and Tanjiro come to blows with the Tsuzumi Demon controlling the house. They're both eventually transported away while Zenitsu and Shoichi are chased throughout the house by the Tongue Demon. While asleep, Zenitsu showcases his incredible Thunder Breathing abilities. At the same time, the wild boar shows off his self-taught beast breathing techniques against the Horned Demon.
Tanjiro and Teruko locate Kiyoshi and learn the demons wish to eat him because he possesses rare "marechi blood". Tanjiro and Kyogai come face to face again in a battle where Kyogai constantly changes the orientation of the room using his Blood Demon Art. Kyogai is also revealed to be a former member of the Twelve Kizuki.
At first, Tanjiro struggles due to the injuries he sustained in Asakusa. After regaining his spirit, Tanjiro is able to adapt to the changes by changing the form of his own water techniques. He sees fit not to trample on Kyogai's writing and even compliments his Blood Demon Art before severing his head. Kyogai dies in peace knowing his talents were acknowledged by someone.
Everyone regroups outside where Inosuke comes to blows with Tanjiro and Zenitsu over Nezuko's box. Inosuke tries to slay the demon without understanding the situation. Even though Zenitsu is in the same boat, he tries his best to protect the box for Tanjiro. Tanjiro had mentioned earlier that this box was more precious than his own life.
Tanjiro and Inosuke get into a ferocious bare-knuckle brawl that ends with Tanjiro giving the wild boar a concussion with his rock-hard head. The Demon Slayers and the children go their separate ways and the new trio recovers at the House with the Wisteria Family Crest. There, they get to know each other and clash personalities. Nezuko also reveals herself and Zenitsu learns the truth about why Tanjiro is traveling with a Demon.