The Demon Slayers vs Muzan Kibutsuj Part 1 is a battle takes place in both the Demon Slayer Corps Headquarters and the Infinity Castle. It is the first major confrontation between Tanjiro Kamado, The Hashiras, and Tamayo against the Demon Lord Muzan Kibutsuji.
Muzan arrives at the Ubuyashiki Estate and confront Kagaya and Amane Ubuyashiki. Muzan expresses his disappointment in the Oyakata; he has lived for a millennium without aging, where as the Ubuyashiki family is reduced to their leader looking and smelling like a corpse. Kagaya doesn't deny this but states his goal is to dedicate his life into defeating him. He states that unknown to him, he and him were once part of the same family. Muzan doesn't understand the significance, so he explains the reason for his family's curse is that one of their members - Muzan himself - became a demon. Since then, children born to the Ubuyashikis died during birth until they were informed that they could atone and survive by dedicating themselves to ending Muzan.
Muzan wonders if Kagaya's rambling is due to his illness and claims he is not punished, having killed thousands of people without suffering. Kagaya suddenly asks for what Muzan dreams about; unsettled, the demon responds by saying he dreams to be immortal and will do so when he takes Nezuko. Kagaya tells his belief that only human thoughts are truly immortal, then continues that the murders he committed over his life only serve to fuel the anger of those who hate him; the Demon Slayers. Kagaya tells Muzan his kind will end tonight, shocking him. He angrily tells Kagaya to silence himself, but he states one more thing anyway. He believes he is undeservingly loved by his Hashira, declaring that his death would only drive them further into killing him. Muzan asks if he is finished, to which Kagaya thanks him for listening. At the same time, a Kasugai Crow calls for all the Hashira be summoned at the Kagaya estate, forcing everyone to run to the mountain.
The Demon Lord is disturbed by his time on the mountain, wishing he was back at his fortress to work on something else. He internally comments on the Ubuyashikis' weirdness, earning Kagaya's ire. He tells Muzan he himself graduated at the top of his class, and as the head of missions against the Twelve Kizuki is responsible for 300 demon deaths. He also warns that the Hashira will be arriving soon and he will be engulfed in a storm that will kill him, directly threatening Muzan and angering him.
Suddenly, the mountainside is destroyed in a massive explosion, wiping out Kagaya, Amane, their two daughters present, and destroying Muzan's body. The Hashiras are horrified at the blast and race to the remains of the estate. Muzan curses Kagaya for sacrificing himself as a decoy, as he never though he was willing to kill himself and his family just to kill him. As he condemns him for hiding his blood lust all along, he realizes the Demon Slayers must now be aware of his presence.
As Muzan attempts to heal the damage, flesh seeds appear around him, bursting into bramble branches that immobilize him in place. Believing he can simply absorb them in, he is suddenly attack by Tamayo, who pierces her hand through his stomach and injects a drug. She tells him he has now been injected with a drug developed by herself and Shinobu Kocho that can turn demons back into humans. Muzan is furious at Tamayo's intervention and reminds her that it was she, not him, who killed her family. Tamayo tearfully cries out that she wanted to live and see her children survive, but is also reminded she herself killed people to survive. To this, she states that they will atone for those human deaths together and calls out for assistance.
Suddenly, Gyomei Himejima appears and decapitates Muzan with his flail. But true to Kagaya's prediction, Muzan regenerated his head with little to no difficulty. Gyomei discerns from the sound of his flesh regenerating that Muzan is unlike any other demon he has ever faced. Recalling his master's words that sunlight is now the only option left, he resolves to keep him restrained till dawn arrives. Muzan uses his Blood Demon Art: Black Blood, Brambles at the Stone Hashira, to which he reflects with the Stone Breathing, Third Form: Stone Skin. Sanemi sees the Demon Lord and screams, asking what he did to Kagaya, while the others arrive, worried for Kagaya. Gyomei announces to the Hashira that the Muzan is the demon who infiltrated their headquarters, and that he will not die no matter how many times he's beheaded. This greatly shocks all the Hashira beyond measure due to them never seeing Muzan in the flesh. Tanjiro however, the only one to have encounter him, reacts with fury seeing his face again.
All the remaining Hashira use their respective Breathing Styles in order to cut Muzan down: Muichiro Tokito with the Mist Breathing, Fourth Form: Shifting Flow Slash, Shinobu Kocho with the Insect Breathing, Butterfly Dance: Caprice, Obanai Iguro with the Serpent Breathing, First Form: Winding Serpent Slash, Mitsuri Kanroji with her Love Breathing, Fifth Form: Swaying Love, Wildclaw, Giyu Tomioka with the Water Breathing, Third Form: Flowing Dance, and Sanemi Shinazugawa with the Wind Breathing, Seventh Form: Gale, Sudden Gusts. Tanjiro Kamado joins in the fight and uses the Hinokami Kagura: Sunflower Thrust. All the Demon Slayers converge on an immobilized Muzan. But instead of being concerned, Muzan gives off a disturbing smile.
Doors open beneath all of the Demon Slayers, causing them to plummet into the Infinity Castle. Muzan mocks the Demon Slayers for believing they had him cornered, declaring they are in hell now and that he will slaughter them all tonight. Tanjiro refutes and says that it is Muzan who will arrive in hell, to which he scoffs and dares Tanjiro to try if he can.
Shinobu, cut off from everyone else, walks through the castle and suddenly smells blood. She walks into a large water-filled room with lilies and wooden planks and enters. Inside she sees someone with multiple female corpses all around him; the demon hears the door creak and turns around to meet his new visitor.
Tanjiro and Giyu fall side by side, Tanjiro nearly falling to his death only to be saved by Giyu catching him. Tanjiro thanks the Hashira but senses a demon creeping up behind him, quickly killing it with the Water Breathing, First Form: Water Surface Slash. Multiple other demons follow requiring both to use the Water Breathing, Sixth Form: Whirlpool and Water Breathing, Third Form: Flowing Dance. Tanjiro venerates Giyu for his skills on using Breathing Styles and can tell which move he does just from the slightest movement he did so he may also use another move without cutting each other.
Obanai and Mitsuri fell together, with Obanai using the Serpent Breathing, Fifth Form: Slithering Serpent to kill a plethora of demons. While he threatens the demons to stay away from Mitsuri, she gushes over how strong and great Obanai is with him asking her if she is okay. Sanemi singlehandedly kills hordes of demons, angered at his inability to protect Kagaya and swears to slaughter them all. Gyomei and Muichiro are together killing demons, to which Gyomei stated that each of the demons have the same power of a Lower Rank of the Twelve Kizuki, believing Muzan is sending them en masse to wear them down.
Inosuke, Genya and Zenitsu all enter the Infinity Castle with their own agenda, as well to assist Tanjiro and the Hashiras. Elsewhere, trapped in Muzan's flesh cocoon, Tamayo calls out for help.