To Defeat Muzan Kibutsuji

This is the fifty-sixth episode of the anime and the first of its fourth season. The Wind and Serpent Hashira, Sanemi Shinazugawa together with Obanai Iguro, pursue a kidnapper to a ruined castle teeming with demons and briefly fall into the Infinity Castle before it vanishes. Tanjiro recovers at the Butterfly Mansion. The Hashira then gather at the Ubuyashiki estate, where Muichiro explains the conditions that triggered his mark. To prepare the entire Corps for the coming clash with Muzan, the Hashira Training begins.

Sanemi and Obanai cut through a swarm of demons inside the abandoned castle and chase a demon that drops the pair into the shifting halls of the Infinity Castle, which collapses and disappears before they can act. Kanao returns to the Butterfly Mansion, where Shinobu recaps the village battle and leaves her to watch over Nezuko. Tanjiro and Goto discuss the relocation of the Swordsmith Village and Nezuko's new immunity, which Tamayo is studying.

At the emergency meeting, Amane Ubuyashiki appears in Kagaya's place and reveals his failing health. She presses Mitsuri and Muichiro to explain the marks once borne by the Sun Breathing swordsmen who nearly destroyed Muzan. Muichiro recalls that his mark surfaced when his heart rate passed two hundred beats and his temperature exceeded thirty-nine degrees. Gyomei Himejima proposes the training program, and the Hashira agree. Zenitsu returns to the mansion and is crushed when Nezuko greets him as Inosuke, while Hotaru delivers Tanjiro's new black blade bearing Rengoku's tsuba. Each Hashira's training stage is introduced, and a crow invites Tamayo to join the Corps headquarters.

The episode adapts chapters 128 through 131. The opening mission that uncovers the Infinity Castle entrance is anime-original, as are Kanao's return, Tengen explaining each training stage to his wives, and the scene of Kagaya thanking Amane. The fourth eyecatcher recreates the cover of chapter 129. Rain is added when Amane mentions the caveat to awakening the mark.