Tanjiro and Inosuke face off against the father of the Spider Demons of Mount Natagumo.
Tanjiro unleashes an attack, only to get hurled away by the demon’s overwhelming power. Landing near a river, he comes upon a young demon Rui torturing his demon sister. Hearing Rui call their bonds of terror and hatred “family ties,” Tanjiro is enraged.
So begins the battle between Rui and Tanjiro.
Upon reaching Mount Natagumo, Shinobu and Giyu come across the corpses of dozens of Demon Slayers. She asks if he was acquainted with any of them, seeing him so quiet, and believes there are no survivors. Shinobu adds that they were advised that several Mizunotos have joined the battle but believes them to be dead now. As they travel through the forest, Shinobu tries to comment on the full moon's appearance; Giyu ignores her, causing her to say that they should at least get along being on the mission together. He retorts he is only here to kill demons. Ribbing him for acting so callously, Shinobu nonetheless agrees; they decide to split up, with Shinobu branching away to begin investigating from the west, while Giyu continues forward.
Father of the Spider Family emerges from the forest to kill the intruders of the mountain. He attempts to punch Inosuke but Tanjiro intervenes with Water Breathing, Second Form: Water Wheel, cutting into his forearm. However, the demon's arm is too hard, leaving Tanjiro stuck in place and unable to slice through. Father tries to punch him with his free arm but Inosuke slices into it to prevent him from attacking. Father flings them away simultaneously; Tanjiro lands on a nearby boulder and tries to think of a new solution.
The demon runs towards him, yelling at the Demon Slayer to stay away from his family and destroying the boulder he was atop of. Inosuke leaps from behind and tries to stab Father, only for the demon to punch him away with incredible force. Hurt by the impact, Inosuke mutters that he isn't at full power and feels the bleeding of his wounds is worsening his state. He runs away and is chased by Father; Tanjiro sprints alongside the river and uses Water Breathing, Second Form: Improved, Lateral Water Wheel. He slices a nearby tree and it falls on top of Father, pinning him down. Initially confused, Inosuke sees it was Tanjiro who had done so, amazed at his skills.
With the demon pinned to the riverbed, Tanjiro sees his chance to slice its neck with the final and most powerful form. He breathes in to strengthen himself, as Inosuke watches in anticipation of seeing something incredible. He runs towards Father and uses Water Breathing's Tenth Form, only for Father to block his blade using the fallen tree. With Tanjiro stuck to the trunk, the giant demon flings him far away; Inosuke calls out to "Kentaro" and Tanjiro begs him not to die till he returns. As he sees his friend hurtling away in the distance, Inosuke is left to battle Father alone.
Atop the suspended house, Zenitsu's condition worsens as he continues his attempts to slow the poison. As he struggles to breathe, he becomes unable to feel his arms and legs, as they have withered away. Down below, the transformed Demon Slayers look at the house, worried for their saviour's safety. Zenitsu apologizes to Nezuko; as he slowly loses the battle against the poison, he remembers his trainer's words to "Don't give up", giving him the hope to continue. Suddenly, something enters his vision; Shinobu lands down beside him and asks if he is alright.
After being hurled a great distance into the forest, Tanjiro uses Water Breathing, Second Form: Water Wheel to slow down his momentum, allowing him to hit the ground without severely injuring himself. Nonetheless, he slams into a tree. As he collects himself, he hears a cry of pain and walks towards it, the voice begging someone to, “Please stop!”. He crouches nearby and watches the situation. Daughter appears on the ground, bleeding heavily and crying, her face slashed by Rui’s webs in his hands. Rui looks to his side and asks why Tanjiro is watching, stating that their debacle isn’t a show. Tanjiro asks if the two are allies; Rui claims their bond is closer than just allies; they are family.
He states he and Daughter have a “powerful bond" and their situation is just between them; he tells Tanjiro not to interfere, “or I’ll carve you up”. Tanjiro claims the demon is wrong; whether a bond is between allies or family, they are equally precious, retorting his statement that a bond between allies is flimsy. He claims that those with a strong bond have the scent of trust, yet all he can sense between him and his sister is terror, hatred and repulsion. He flatly calls their bond forged, surprising Daughter and enraging Rui.
At that moment, a Demon Slayer appears. He sees Rui and determines the child-like demon to be an easy kill, seeing him as an easy way to climb the ranks and get better pay. He charges at Rui but the demon flings a web at his attacker, eviscerating his body instantly. As Tanjiro tries to comprehend the sudden and gruesome death before him, Rui turns to Tanjiro and menacingly asks what he had just said.
Tanjiro feels the fearsome intimidation the demon is giving off and apologizes to Inosuke, begging him to stay alive and promising to assist him after his new battle. Rui demands he repeat his statement; undaunted, Tanjiro tells him he will say it over and over again, calling their bond forged. Elsewhere, behind a tree, Inosuke catches his breath and watches as Father looks around the forest for him. The giant demon sees a trail of his blood and Inosuke realizes he has been spotted.
As Father approaches him, Inosuke tries to think of a plan, barely escaping when the spider demon destroys his cover. As he runs away, he tries to rationalize how he can stay alive till Tanjiro's return. He suddenly stops in his tracks, thinking about how he is retreating since that isn't who he is. He turns back and charges towards Father; leaping into the air, the Demon Slayer slams his right blade into the demon's forearm. Annoyed that his way of thinking is becoming like Tanjiro's, Inosuke hammers his second blade into his first and successfully slices off Father's forearm. Satisfied at causing real damage, Inosuke raises his swords at his opponent. Without warning, Father runs away, to the surprise of the Demon Slayer; Inosuke demands he come back to face him and runs after the demon.
Tanjiro attacks Rui but is forced back by the demon's numerous threads, several of which have managed to slice Tanjiro's face. Rui declares his intentions to torture him slowly, but offers an instant death if he takes back his words. Tanjiro vehemently refuses and bluntly states he is the only one not making sense, to Rui's silent fury. Daughter watches in shock as the Demon Slayer challenges him.
Running through the forest, Inosuke attempts to find Father. He kneels down from the pain of his injuries but attempts to brush them aside and keep going. Using Spatial Awareness, albeit with some trouble from his injuries, he locates the demon hiding up a nearby tree. Believing the monster plans to "make me use my head again", Inosuke then watches as Father begins to shake uncontrollably. Thinking the demon is shaking in fear at the sight of him, he is proven to be mistaken when the outer layer of the demon's skin sheds away, revealing a far more powerful form. Father lands down to meet his opponent; as Inosuke stares in shock at his opponent's new body, he becomes overwhelmed with fear and doubt.
Suddenly, he thinks back to Tanjiro's last words to him, "Don't die until I get back!", and Hisa's words of advice to "Always live your lives with great pride". Motivated to keep fighting, Inosuke declares he is "Inosuke Hashibira of the Demon Slayer Corps" and raises his swords to fight. Father suddenly punches him square in the body, sending him flying into several trees and slamming into the ground. The demon quickly finds him and tries to kill him; Inosuke leaps high into the air to dodge the blow and uses Beast Breathing, Third Fang: Devour as he descends. When he tries to slash at Father's neck, his blades snap upon making contact. In shock, he fails to dodge Father's next attack and is violently punched into a tree.
Father grabs him by the head and yells at the Demon Slayer to stay away from his family. As he begins to crush Inosuke's head, Inosuke desperately uses Beast Breathing, First Fang: Pierce with his broken swords. The tips stab into Father's neck but the demon remains unfazed. Father holds him outstretched, separating Inosuke from his swords, and slowly crushes him. As his life flashes before him, Inosuke sees a woman apologizing to him and recalls his friends and Hisa; the woman wishes for him to survive as she stands on top of a cliff, calling out to him. Inosuke asks who she is but blood pours out of his mask.
Suddenly, something speeds into the battle and slices off Father's arm, freeing Inosuke. Giyu stands before Inosuke, who asks if he had slashed the demon, staring in wonder at the new arrival. Father regrows his arm and charges at the Demon Slayer; unafraid, Giyu uses Water Breathing, Fourth Form: Striking Tide and instantly slices the giant demon to pieces. In awe at what he had just witnessed, Inosuke comments he has never seen someone as powerful as him.
As Tanjiro dodges the threads coming towards him, he senses the odour of the forest is clearing, allowing him to sense the threads once again. Rui notes that his opponent is smarter than he thought and isn't afraid but sees it as irrelevant. Tanjiro runs towards him and leaps into the air; Rui launches a thread at him and Tanjiro responds with Water Breathing, First Form: Water Surface Slash. As his sword makes contact, the thread slices through the steel and continues its way towards Tanjiro.
Characters in Order of Appearance