In Episode 21 of "Dark Gathering," the story begins with the young spirit eating meatballs made from Shami's remains. Yayoi, who knows the S-rank of the spirit, advises Keitaro and Eiko to be cautious and explains the young spirit's powers and the nature of his curse. She emphasizes the importance of destroying his sketchbook to stand a chance against him.

As Yayoi devises a strategy to seize the sketchbook, she is suddenly attacked by ghostly hands and dragged against a railing. Keitaro tries to help but is overwhelmed by more hands. He urges Eiko to save Yayoi first. Eiko assists Yayoi with a tool that allows her to escape, and they refocus on rescuing Keitaro.

In a surprising twist, Eiko finds herself trapped in a confined space with a male and a female spirit. Yayoi's timely intervention, aided by the spirit in Eiko's doll, saves her from this danger. The two spirits flee to the young spirit and pledge their loyalty to him. Yayoi rescues Keitaro, and Eiko manages to snatch the sketchbook from the young spirit.

Yayoi's taunting provokes the young spirit into attacking Otogiri Oiran, a courtesan Yayoi summons to fight the boy. After Yayoi tells Otogiri the boy's story, the two spirits engage in a fierce battle. Yayoi realizes that the boy's new armored form poses a significant challenge.

Amid the chaos, Otogiri releases butterflies, and Yayoi warns Keitaro and Eiko to stay away from them. She explains the purpose of the butterflies while Otogiri speaks to the young spirit. Otogiri's second curse activates within the young spirit, and Yayoi reveals her plan to drain his essence.

The enraged young spirit turns dead bodies into meatballs to grow stronger and attacks Otogiri, who counters with life-draining butterflies. Yayoi, aiming to trigger Otogiri's third phase, uses a mirror to reflect her current state.

The episode then sheds light on Otogiri's past, particularly her relationship with her apprentice Kurenai and her lover Ito. Kurenai's betrayal is revealed, leading to a dramatic confrontation where Otogiri sets her surroundings ablaze. The episode ends with the activation of Otogiri's third curse, intensifying the battle.