Tanjiro, Zenitsu, and Inosuke go on their first mission together and venture to Mount Natagumo.
Too afraid to go up the "spooky mountain" with Tanjiro and Inosuke, Zenitsu stays behind. Tanjiro and Inosuke encounter a mountain laced with spider webs and controlled by Spider Demons. These Demons have already dispatched countless Demon Slayers, forcing Kagaya to send in the Hashira as reinforcements.
Zenitsu ascends the mountain after realizing Tanjiro has taken Nezuko into danger. After convincing Inosuke to work together, Tanjiro and the wild boar defeat the Mother of the Spider Clan. Zenitsu encounters the brother and hones his abilities to the utmost limit. Despite getting poisoned, Zenitsu bests the Spider Demon alone.
As Tanjiro tries to treat Inosuke's wounds from battle, they encounter the Sister spider and then her Father. The Father demon separates Tanjiro from Inosuke and takes on the lone boar. Meanwhile, Tanjiro encounters Rui, the creator of the clan.
Giyu alias the Water Hashira arrive with his companion Shinobu Kocho, the Insect Hashira, on the Mountain and rescue both Zenitsu and Inosuke. Tanjiro calls the bonds between the Spider Family fake, enraging Rui. They engage in a fierce fight where Rui intends to kill Tanjiro and make Nezuko his own sister.
Refusing to sever their own bonds, Tanjiro and Nezuko fight back and nearly defeat the Demon with newly awakened abilities. Tanjiro unlocks his ability to use his father's Hinokami Kagura dance and Nezuko unleashes her very own Blood Demon Art.
However, Rui survives the battle and nearly kills them both. Giyu rescues the siblings in time and effortlessly defeats Rui. At the same time, Shinobu dispatches the Sister of the family, finishing off the Demons. Rui regains his human memories and regrets his actions before entering the afterlife, where he reunites with his family.
Shinobu attempts to kill Nezuko because she's a demon but Giyu defends the siblings. Tanjiro tries to escape with Nezuko but they're caught by Shinobu's assistant, Kanao Tsuyuri. Shinobu warns Giyu and Tanjiro that they're both going against corps rules by aiding a demon. Before things escalate, Kusagai Crows announce that both siblings are to be taken into custody.