In Episode 6, Tanjiro and Zenitsu begin their training under the supervision of the Stone Hashira. The grueling tasks include enduring a cold waterfall, lifting heavy logs, and pushing a large boulder over a distance of about 109 meters. Zenitsu collapses from exhaustion and is thrown into the river on the Hashira's orders, where the cold water quickly revives him, and he starts loudly complaining.

Tanjiro is inspired by Inosuke, who bravely stands under the waterfall until he too passes out. Tanjiro immediately tries to revive him before taking on the harsh waterfall training himself, only to end up lying on the rocks with the others, warming their frozen bodies.

During lunch, the students discuss the Stone Hashira's strength, with Tanjiro mentioning his unique scent. In the following days, Zenitsu and Tanjiro continue their intense training, though Tanjiro struggles to move the boulder despite all his efforts.

In the Infinity Castle, Muzan uses a fourth-ranked demon with multiple eyes to spy on the Demon Slayers and locate Nezuko and Urokodaki.

Back at the training grounds, Tanjiro keeps trying to move the boulder and cooks for the students during breaks. Genya stops by and explains the technique of repetitive action, which he learned from the Stone Hashira. This technique, similar to Total Concentration Breathing, involves specific movements like reciting or recalling intense emotions to enhance physical abilities.

Genya also notes that Tanjiro's mark is darkening, which is a positive sign. As Tanjiro practices this technique, he remembers his family and Rengoku's words: "Set your heart ablaze!" This motivates him to heat up his body and slowly move the boulder while his friends watch in amazement. Zenitsu remarks in awe, "You're a monster."