In the wake of Nezuko conquering the sun, the battle against the demons reaches a fever pitch. Knowing that Muzan Kibutsuji will stop at nothing to claim her, the Demon Slayer Corps initiates the "Hashira Training"—an grueling, high-intensity regimen designed to enhance the abilities of every slayer and manifest the "Demon Slayer Marks" among the Hashira.

Tanjiro and his fellow slayers move through a series of increasingly difficult camps led by the elite Hashira, including the eccentric Sound Hashira Tengen Uzui, the strict Wind Hashira Sanemi Shinazugawa, and the enigmatic Stone Hashira Gyomei Himejima. As the slayers push their bodies to the absolute limit, the season shifts from a training arc into a high-stakes psychological standoff, culminating in a devastating surprise attack by Muzan himself that sets the stage for the final war within the Infinity Castle.