The plot of the film focuses on the Demon Slayers' entry into the Infinity Castle and several decisive battles against the Upper Ranks of Muzan's Twelve Kizuki.

The Demon Slayers, including Tanjiro Kamado and the Hashira, are lured by Muzan Kibutsuji into the Infinity Castle, a dimensionally-shifted, labyrinthine fortress that constantly shifts and whose structure is controlled by the biwa-playing demon Nakime. The castle defies conventional spatial logic, with rotating corridors, varying gravity, and infinite levels, serving as a protective space for Muzan and his Upper Ranks. The Demon Slayers are separated upon entry and must navigate this hostile environment while simultaneously being attacked by numerous lower-ranking demons deployed by Muzan as a distraction and weakening strategy.

Zenitsu Agatsuma encounters Kaigaku, a former fellow student and mentor who joined Muzan and now holds the rank of Upper Six. Their battle is emotionally charged by betrayal and a shared past: Kaigaku was complicit in the suicide of their master by betraying his legacy. Zenitsu, who previously only mastered the first form of Thunder Breathing, stakes everything on a self-developed technique, the Seventh Form: Flaming Thunder God (God of Fire Thunder), with which he ultimately beheads and defeats Kaigaku.

Shinobu Kocho, the Insect Breathing Hashira, confronts Doma (Upper Two), the murderer of her sister Kanae. Since Shinobu is physically too weak to sever Doma's neck, she pursues an elaborate poison strategy: she intentionally allows herself to be absorbed by Doma to weaken him from within with a highly potent poison specifically developed for high-ranking demons. Although Doma initially shows resistance to her poisons, Shinobu succeeds in significantly weakening him, but sacrifices herself in the process.

Tanjiro and Giyu Tomioka (Water Breathing Hashira) jointly face Akaza (Upper Three), one of Muzan's strongest demons. During the battle, Tanjiro reaches a new level of consciousness, the Transparent World, which allows him to foresee Akaza's movements and neutralize his attacks by suppressing his fighting spirit. Akaza's suppressed human past emerges: flashbacks show his development from a human fighter named Hakuji, who became a demon through tragedies and Muzan's influence. After being beheaded by Tanjiro, Akaza refuses to regenerate and destroys himself, overwhelmed by guilt and inner conflicts.

Parallel to the individual battles, Kiriya Ubuyashiki, the leader of the Demon Slayers, coordinates the troops from the outside using crows that serve as communication and reconnaissance means. The ultimate goal remains the hunt for Muzan Kibutsuji himself, who is hiding in the core of the castle and aims to defeat the Demon Slayers through exhaustion and the Upper Ranks. Muzan also plans to absorb Nezuko's ability to withstand sunlight to become invincible. The Demon Slayers must find Muzan before he fully recovers or can assimilate Nezuko's blood.