One of the Twelve Kizuki, a direct subordinate of Muzan Kibutsuji, is here somewhere. When Tanjiro slays the Mother Spider Demon of Mt. Natagumo, he learns that a demon who might hold the key to turning Nezuko back into a human is somewhere in this very forest. With the battered Inosuke in tow, he heads deeper into the forest. Meanwhile, Zenitsu trudges forlornly through the woods only to encounter a spider with a human face.
Zenitsu flees from a spider with a human head only to find the nest of the demon that made it, Father's elder son, whose venom turns humans into spiders. He reveals to Zenitsu he was bitten by one of his spiders during his wandering and will lose his human form in thirty minutes. Zenitsu hides in a tree as other demon-spiders approach him.
He remembers his days of training under Jigoro Kuwajima, who paid off his debt if he became his apprentice. He was struck by lightning while hiding from his training in a tree. He thinks to himself he is a pathetic person and wants to stop being such a sniveling coward. When Zenitsu realizes his hair is starting to fall out, he faints.
While asleep, he starts to attack the elder son. He remembers how he learned only one attack of the Thunder Breathing, but mastered it so well he perfected it. He remembers being bullied by Kaigaku, another student of Kuwajima, who considered time spent on him a waste. Being an orphan, Zenitsu had no one who dreamed he would make something of himself, no one who had a reason not to abandon him, and everyone did, except Kuwajima.
The poison starts to affect Zenitsu, making him cough up blood, but he finishes the attack and, traveling at lightning fast speeds, corners and slays the elder son with the Thunder Breathing, First Form: Thunderclap and Flash, Six Fold.
He wakes afterwards but the poison has crippled his ability to move. Though still scared, Zenitsu uses a breathing technique to slow the poison as much as he could.
Tanjiro hears Zenitsu's attack and decides to head to it. Before he and Inosuke part ways they come across the Daughter of the spider family, who flees when Inosuke chases her and calls for Father. Father comes to her aid and the Daughter leaves. Father presses the attack on the two causing Tanjiro to retaliate.
- 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 mutsu?) into "3" (三つ mitsu?).