Shortly after defeating Gyokko, one of the Upper Five Kizuki, Mist Hashira Muichiro Tokito experiences the emergence of a mysterious mark on his face, which significantly boosts his strength and speed. However, this newfound power comes at a cost, as he soon succumbs to exhaustion, needing assistance from his fellow swordsmiths, Kozo Kanamori and Kotetsu.
Meanwhile, Tanjiro Kamado [Natsuki Hanae] and Genya Shinazugawa find themselves aiding Love Hashira Mitsuri Kanroji in a fierce battle against the Upper Four, known as Hantengu. To evade capture, Hantengu shrinks himself down, while his manifestation of hatred, Zohakuten, relentlessly attacks the duo with a Blood Demon Art that conjures giant wooden dragons. In a display of courage, Mitsuri unlocks her mark to confront Zohakuten directly, while Genya and Tanjiro pursue Hantengu. Tanjiro channels his inner strength, utilizing his "Hinokami Kagura" techniques to strike a critical blow to the demon's neck. Just when victory seems near, Hantengu releases his representation of resentment, known as Urami, who almost claims Tanjiro's life.
In a climactic moment, Urami emits a sonic shriek that incapacitates both Genya and Tanjiro. However, at this crucial juncture, Nezuko, Tanjiro's demonic sister, arrives to save the day. With a splash of her blood, she ignites Urami in flames. The two heroes plummet down a cliff where they are met by their allies Muichiro, Kozo, Kotetsu, and blacksmith Hotaru Haganezuka. In a final act of selflessness, Muichiro hands Tanjiro Hotaru's unfinished sword while dawn breaks on the horizon.
Due to Zohakuten's excessive use of their combined power against Mitsuri, Urami discovers the three swordsmiths concealed in the distance and makes a beeline for them. In a desperate move, Tanjiro severs Urami's head, only to witness in horror as Nezuko begins to burn in the harsh sunlight. Realization dawns on Tanjiro as he sees Urami's beheaded body still moving; the true form of Hantengu remains unscathed. Paralyzed by indecision, Tanjiro is ultimately encouraged to act by Nezuko, who signals it is time to eliminate the demon.
With clarity of purpose, Tanjiro understands that Hantengu is hiding in the core of Urami. He slices Urami in half and exposes Hantengu to the unforgiving sunlight. As Hantengu faces his demise, he reflects on his former life as a criminal; in contrast, Zohakuten disintegrates, granting Mitsuri the chance to survive. Although Tanjiro mourns what he believes to be Nezuko's death, a miracle occurs—she is not only alive, free from her fiery plight, but also begins to speak once the bamboo mouthpiece falls away.
Through the fragmented memories of Hantengu, the demon king Muzan Kibutsuji uncovers that Nezuko possesses an immunity to sunlight, and he quickly realizes that his aspirations for immortality hinge on devouring her. In the aftermath of the intense battle, Muichiro expresses gratitude towards Tanjiro for helping him reclaim his identity. The Love Hashira, Mitsuri, embraces both, celebrating their survival as the village rejoices in the victory of the siblings.
One week later, Zenitsu Agatsuma [Hiro Shimono] and Inosuke Hashibira [Yoshitsugu Matsuoka] return from their respective missions. Tanjiro, still in recovery from his injuries, is gifted a new sword adorned with the late Flame Hashira Kyojuro Rengoku's tsuba by Hotaru. Additionally, Tanjiro receives a letter from Tamayo, indicating her successful experiments with blood samples from the Twelve Kizuki aimed at reversing demon transformations back into human form.
In a separate mission, Serpent Hashira [Obanai Iguro] and Genya's elder sibling, Wind Hashira [Sanemi Shinazugawa], delve into an abandoned temple that harbors demons. They quickly uncover that the site serves as one of several entry points to the Infinity Castle, Muzan's vast, extra-dimensional stronghold.
As the leader of the Demon Slayer Corps, Kagaya Ubuyashiki, nears his last days, his wife Amane gathers the Hashiras for a momentous meeting. Here, Muichiro and Mitsuri share the harrowing tale of their mark awakenings, both occurring under extreme stress. They highlight the importance of a unique training regimen to harness this newfound power and equip the Corps to confront Muzan's impending arrival for Nezuko. Amane assigns Insect Hashira [Shinobu Kocho] to collaborate with Tamayo in developing a concoction effective against Muzan while Stone Hashira [Gyomei Himejima] spearheads the training initiative, with retired Sound Hashira [Tengen Uzui] called upon for assistance. Meanwhile, Water Hashira [Giyu Tomioka] decides to forgo participation, committing himself to solitary training instead.
