Episode 17

You Must Master a Single Thing (ひとつのことを極め抜け) is the 17th episode of the Kimetsu no Yaiba TV anime series.

Synopsis
The Twelve Kizuki, directly subordinate to Muzan Kibutsuji, are here…When Tanjiro slays the mother of the Spider Demons of Mount Natagumo, he learns that a demon who might become the key to turning Nezuko back into a human could be in this very forest. Despite how battered Inosuke is, he heads deeper into the forest with Tanjiro. Meanwhile, Zenitsu trudges forlornly through the woods, only to encounter a human-headed spider.

Plot
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.

Tanjiro attempts to patch Inosuke's wounds, to his annoyance, and ponders on the strange nature of the mountain, as it has both a Twelve Kizuki member and a demon family within, despite the fact that demons don't group together. 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 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 that, "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. The demon tells Zenitsu he will soon be his slave inside of 30 minutes, commenting he will feel pain, dizziness, nausea, lose consciousness and then become a spider, laughing as he taunts the Demon Slayer over his impending fate. Now even more frightened, Zenitsu runs away and is chased by numerous transformed spiders. The demon reminds him that running is futile, to which Zenitsu yells he already knows, climbing up a tree to avoid the spiders. The spider demon tells him not to worry, as his intelligence will be reduced too. Zenitsu refuses to listen and laments he has no friends, a girlfriend or anyone that likes him; hearing this, the spider instead offers an extra dose of poison to accelerate his transformation. Zenitsu becomes frightened at his fate and hugs the tree, reminding him of when he had climbed up a tree to escape his trainer, admonishing him for running away continuously. His trainer commands him to climb down to resume training but Zenitsu adamantly refuses, sure that he will die by continuing. His trainer tells him he won't at the level they will be at and tells him again to get down. Zenitsu refuses, calling him as "Gramps", his trainer commands to be addressed as "Master"; Zenitsu then replies it is because he loves him, much to his trainer's surprise.

Zenitsu recalls when he first met him; he was with a girl who made him raise money just to start an affair with a different person. Racked with debt, he was rescued by his trainer and subsequently taught to be a Demon Slayer. Though he aspired to meet his expectations, he didn't believe himself to be capable of reaching them. He states that he does regret being so weak-willed and cowardly, adding that he had also been training in secret without getting sleep, yet he still is yet to show any amount of progress. As he laments his inability to do anything, thunderclouds roll in. His trainer attempts to calm him down but as he continues ranting, a lightning bolt strikes the tree, hitting Zenitsu. Recalling the moment as he fell down, his hair turned yellow by the lightning, he reflects on the tragedy that is his life. He was thankful just to be alive, he hated himself more than anything, and he always thought he could hold it in but would always fall apart emotionally nonetheless. Still reflecting on what brought him to this moment, Zenitsu knows he would always do everything he can, angrily asking why he has to turn into a spider now.

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 mocks his cowardice, expressing disbelief he was sent to kill his family.

As his body slumps from the tree and plummets, Zenitsu begins to breathe. Before he can hit the ground, he twists himself upright and plants his feet against the tree, preparing to attack. He launches himself at the Spider Demon, who retaliates using Poison Sputum, hurling 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. In a previous training session, Zenitsu becomes despondent for only being able to do the first form, but his trainer sees this as cause for celebration, telling the young man he can hone it. Recalling this moment, he remembers that he was initially angry because Thunder Breathing has six forms. His trainer then began hitting his head repeatedly; as he does so, he explains that swords need to be struck continually to remove impurities, to give them better durability. Zenitsu sees this as why he continues to clobber him to improve but states that he isn’t made of steel. As time continues to tick down, the Spider Demon taunts him for calling himself a Demon Slayer despite doing just one form. As his hand begins to shake, Zenitsu remembers his trainer’s encouragement, telling him it is okay to cry and run; instead, he asks that he never give up and to believe in himself. His trainer tells him he can be the most durable blade of all and to hone his form to perfection, as electricity crackles from Zenitsu’s hand. Launching another Poison Sputum blob, Zenitsu dodges it in time but sees himself surrounded by the spiders. As they converge on him again, he remembers another pivotal moment during his training; a fellow student had angrily dismissed him, frustrated by his crying and cowardice. He states their trainer was an extraordinary man and time spent with him is being wasted; when Zenitsu tries to talk about him, referring to him as “Gramps”, the student rebukes him for acting familiar with him, a respected former Hashira. Zenitsu reflects on his life; as an orphan, he had no one to give him hope for the future, no one to be proud of him for doing good. No one believed he could succeed, as his cowardice made people scornful of him, no one but Gramps. His trainer held on and continued to push and reprimand him. Any time he tried to escape, he was there to take him back. Though he was sometime harsh, Gramps never once abandoned him. 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 thunder cloud, he asked 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 they “Don’t come near my family, stay away!”. As the female demon, Daughter, leaves them to escape, Father prepares to strike a vulnerable Inosuke. Tanjiro responds with Water Breathing, Second Form: Water Wheel, his blade barely able to cut into his arm.

Anime Notes

 * In some English translations, this episode has a translation error that states that the Thunder Breathing has 3 teachable forms, there are however, 6 forms. This may be due to the fact that the letter "i" is right next to letter "u", which may have turned "6" (六つ) into "3" (三つ).
 * The sequence depicting Thunderclap and Flash, Sixfold is much shorter in the manga, showing Zenitsu using the form right after Son had ordered his minions to sting him with more venom. Furthermore, Zenitsu is shown speeding around the forest waiting for the right moment in the anime, while the manga depicts him attacking immediately after using it.
 * In the manga, Zenitsu is attacked by three of Son's minions, while the anime shows him being attacked by nearly a hundred.

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