With a true member of the Twelve Kizuki present on Mount Natagumo, Tanjiro realizes it must possess a higher concentration of Muzan's blood, reaffirming his goal to extract it for the medicine for Nezuko. He runs back to Inosuke, who asks if he had defeated Mother. Tanjiro tentatively answers so and asks if he is alright, noticing his numerous cuts. Inosuke ignores his "showering of sensitivity and kindness," proudly proclaiming he can beat him at anything. Despite this, Tanjiro blames himself for Inosuke's injuries and then remembers Mother's final moments: how she gave off the scent of suffering and terror and when she willingly surrendered.
Elsewhere, Zenitsu feels a prick on the hand as he continues his way around the forest. Now angry at being lost for so long, he wonders about Tanjiro and Inosuke's whereabouts; he finds a two-way fork at the road and becomes more frustrated, as well as annoyed at the terrible smell present and numerous spiders around. With his enhanced hearing, he senses someone behind him and yells at them to stay still; instead, he looks down to see a large spider with a human head.
Terrified, he runs way screaming that he must be in a dream, unwilling to believe what he has just seen. He begs to wake up on Nezuko's lap and plow fields just to escape his "nightmare." As he stops, he finds himself in an enormous clearing in the forest and sees numerous captured humans transforming into spiders. Zenitsu sees a floating house suspended by threads and is further repulsed by the stronger stench of the area, believing it is pungent enough to kill Tanjiro with his sense of smell. He hears a rustling from the house and sees an enormous spider demon emerging from below.
Zenitsu runs away at the sight of the monster, but the demon taunts him by saying, "You're already losing." He tries to ignore his words, but the demon continues talking about his situation becoming dire. Zenitsu asks what the demon is talking about: the spider asks that he look at his hand, and he sees it has become contused and blistered. The spider tells him he was poisoned by a bite earlier and will soon transform into one as well, telling Zenitsu he will soon be his slave inside of 30 minutes and remarking he will feel pain, dizziness, nausea, lose consciousness, and then become a spider, laughing as he taunts the Demon Slayer over his impending fate.
Zenitsu recalls when he first met his master how he trained him even though he is a coward and nuisance, and all his past everybody abandoned him, but his master never abandoned him like others.
Watching the Demon Slayer yelling and crying in the branches, the Spider-Demon grows more confused at his opponent. As the spiders climb up the tree, Zenitsu tells them to stall and let him speak; as he grabs his head in frustration, clumps of hair come off, to his shock and annoyance, as he wasn't told his hair would be gone first. The Spider Demon laughs at his impending doom, causing Zenitsu to pass out in fear and lose consciousness. Stunned to see the Demon Slayer asleep, the demon expresses disbelief he was sent to kill his family.
As his body slumps from the tree and plummets, Zenitsu begins to breathe. He twists himself upright before landing and plants his feet against the tree, preparing to attack. He launches himself at the Spider Demon, who retaliates using poison sputum, spitting a blob of acidic poison at him. Zenitsu dodges in time and lands safely, as the demon notes that he appears and acts completely differently now. As Zenitsu prepares to attack with his Thunder Breathing First Form, the demon signals his spiders to attack him. They converge on him and force the Demon Slayer to repeat his stance. The demon then spits another poison blob, forcing him back again. He notices Zenitsu is repeating the same stance and realizes to his delight that he can only do one form.
The Spider Demon signals his minions to poison him even more; with their barbed tongues, the spiders attempt to envenomate Zenitsu further. As he backs away to safety, he vomits blood from the poison, causing him to stop and allowing the spiders to overwhelm him. As the Spider Demon watches in delight, the spiders are all suddenly blown away as lightning envelops Zenitsu. As the demon lands down, watching the Demon Slayer in renewed fear, Zenitsu uses Thunder Breathing, First Form: Thunderclap and Flash, and extends the attack sixfold.
The Spider Demon uses poison sputum once more, but the poisonous blob is too slow to hit Zenitsu. He speeds around the clearing in preparation to strike, as the demon loses track of the Demon Slayer. Seeing his chance, Zenitsu propels himself at the retreating demon, following it to its house and using one of the threads to launch himself upwards, decapitating the demon instantly. The creature becomes angered at his death at the hands of someone like him, expressing disbelief at the absurdity of Zenitsu's attack while barely able to move his legs and arms before disintegrating to ash.
Zenitsu lands atop the suspended house; as he lies in pain from the poison, he addresses his dream and how he has achieved it; he has now become powerful and can help those in need, as his trainer had taught him. Despite this, he believes his dream to be over with the poison. Chuntaro lands beside his master and tears up. He apologizes to the sparrow, but Chuntaro flies away. Remembering his trainer's words to "Don't ever give up!" Zenitsu uses his breathing to slow the poison's circulation, unwilling to let his trainer "let him have it" and not wanting Tanjiro to be mad.
Elsewhere, Tanjiro hears the sound of a lightning strike in the distance—Zenitsu's attack. Confused since he doesn't smell a thundercloud, he asks Inosuke, who brushes aside his concern. As he is about to cross a river, Tanjiro offers to investigate further and asks if he is descending the mountain. Inosuke asks why he would give up, and Tanjiro points out his injuries. Inosuke irritably denies being hurt, to his friend's bewilderment. At that moment, they spot a pale demon across the river, Tanjiro noting that he was unable to sense her because of the demon scent throughout the mountain.
Inosuke gives chase and pursues her, only for her to suddenly call out "Father!". Wondering what she means, Inosuke is answered by the appearance of an enormous, spider-like demon leaping into the water. As Inosuke and Tanjiro stagger back at the sight of the new threat, Father angrily demands, "Don't come near my family! Stay away!". As the female demon leaves them to escape, Father prepares to strike a vulnerable Inosuke. Tanjiro responds with Water Breathing, Second Form: Water Wheel, but his blade is barely able to cut into his arm.
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 Mizunoto have joined the battle but believe 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 and decides they should split up, with her branching away to begin investigating from the west while he continues forward.