Kaii to Otome to Kamikakushi

Sumireko and Adashino work and chat together in the quiet of the bookshop where they're employed. Their manager appears and gives Sumireko a strange book of unknown origins. Just after midnight, she casually picks it up to read it, and then...

The curse of Tsukuyomi's Water of Youth hasn't finished with Sumireko yet. Meanwhile, Adashino's younger sister, Oto, picks up on the presence of a Curiosity at the school she attends. Sumireko poses as Oto's friend in order to infiltrate the school.

Uname's childhood in the countryside caused her to become a teacher with a strong desire to eliminate bullying. After hearing about the dribblers from Sumireko, Adashino is certain that a Curiosity is involved and comes up with a plan to catch it.

The cuckoo known as Adashino cries in the deserted Midori Bookshop once more. Sumireko intends to spend her day off writing. As she's admiring the sakura outside her window, she gets a text from Adashino asking her to look after Oto for two or three days.

Sumireko looks after Oto while Adashino is away. They go to Oto's school where Sumireko revels in the joys of youth. She and Oto run into Uname and spend the evening with her. But their peace is short-lived as the shadow of a new Curiosity draws near.

Shizuku is looking for Tomoko, who disappeared on a rainy night a year ago. Based on Tomoko's final video call, Adashino has a guess about the Curiosity, but he has a few concerns. Oto wants to help Shizuku, so she begins her own search.

The hong yi xiao nu hai has become a Curiosity which kills the living who answer its knock at the door. But that isn't all. When Shizuku learns the conclusion that Sumireko and Adashino have come to, she steels her resolve and does the unthinkable.

When Sumireko was young, her inability to express herself often led to trouble with her friends. One day, she ran from them and ended up at a strange bookshop. The woman there recommended a book and offered a suggestion to help her speak more easily.

Oto's friend Nodoka dreams of becoming an idol, but as her family runs a hospital, there are certain expectations for her future. Her sole comfort is the VTuber Himeuo Yorumun. But one day...

Yorumun begins hijacking popular streamers' videos, rapidly escalating the commotion around her. Adashino tries to save Nodoka, but Nodoka refuses his help, defending Yorumun instead. Then Uname brings a woman in a wheelchair to meet them.

Adashino and Sumireko discuss the coming of summer and what that means regarding the wearing of swimsuits. Then, as Adashino answers his phone, an otherworldly space envelops the two of them, and Sumireko notices a mysterious figure in the distance.

Adashino has vanished from Sumireko's sight. The Cat Sovereign appears at the station just as Oto is returning from visiting Uname. Adashino intervenes, but speaking with the Cat Sovereign makes Oto remember something she's forgotten for ten years.