The story opens with a colored scene of Marie Moriya getting strangled as she wonders why it turned out like this.
The scene cuts to the past, where Marie is cleaning a classroom with classmates as they fool around. They begin mocking her, thinking she fell asleep while standing up. The classmates leave for cram school, leaving Marie alone to clean up by herself. She finishes cleaning and then feeds the school's bunnies, relaxing that the boys of the committee won't be coming today.
Suddenly, her teacher Kanamori appears and asks if today is fine for her. They go to the old abandoned school building nearby, where Kanamori is holding a camcorder and commenting on how the old building was neglected after they started building and maintaining a new one. He steps into a bathroom stall and begins filming a naked Marie, giving her "supplementary lessons". He leaves soon after, leaving a broken Marie to get dressed and wonder why it all turned out this way.
She thinks back to when Kanamori saved her from bullies and made them apologize, which made Marie so happy at first. One day, Kanamori found her alone in a room after Marie accidentally broke a statue. He tells her that the statue was full of beautiful memories by the principal who everyone loves, and that she'll be bullied by the entire school if word gets out. She cries, and Kanamori says he'll keep her secret if she shows him something more "beautiful". Marie endured all the ensuing sexual abuse and desperately wants it to stop, but can't as long as Kanamori has those videos.
Marie returns home to her apartment and finds a note from her mom asking to order food. She looks off the balcony, presumably about to jump off, and sees how long of a fall it is; she cowers, not knowing what to do.
The next day, Kanamori plays basketball with some kids and the gym teacher, scoring high and getting praise from everyone. Marie's classmates are swooning over him and call it fortunate his his eyes are all over Marie, who gets paranoid with fear over what would happen if they found out about her relations with him. She snaps out of it when she realizes they're just making fun of her for being a bullied kid and getting all the attention, and a girl says she'd rather be bullied too. The other kids then make jokes about how Marie sitting in the corner will get absorbed by it and become a youkai. They start talking about the "seven school mysteries", mentioning that the school nurse said the "doll's complexion" got good. Marie tries to join in the conversation but the girl from earlier shuts her up, saying "youkai don't join in".
Marie heads to the infirmary, where the nurse bandages her hand. Marie sees the doll on her cabinet, and the nurse says it's one of the seven mysteries, supposedly dancing around at night and cursing people who see it. The nurse picks it up and playfully dances it around, remarking that all the students are afraid of it. The nurse then leaves, calling the doll the "master of the school" and that if you say a prayer to it, it will come true. Skeptical of the idea, Marie decides to say a prayer to the doll, wishing to kill Kanamori and everyone in her class for doing bad things to her. She truly thinks about killing them, but stops after realizing that the master will kill her for wishing such a bad thing. The doll seems to move and Marie jumps back in shock, but pays it no heed and leaves as well.
Later that day, Marie and Kanamori are having another session in the old school building, and Kanamori beats her, calling her irreplaceable and that she belongs to him forever. Marie cries to herself, realizing that even if she makes a wish to the master of the school, it won't really work; she must become a youkai and kill them, chewing them to pieces.
The next day, Marie still dreams about becoming a youkai when she hears students talking about the new youkai Mesomeso-san.
A girl goes to the old abandoned school building feeling adventurous, intending to go home afterwards when she hears the voice of a girl sobbing in a women's bathroom. She was afraid of making any noise, believing something bad will happen when the voice suddenly stopped and said "Would you there like to listen to my pitiful story?" The girl became even more afraid and said yes, then the door opened and she was dragged inside by Mesomeso-san, killed by her whole body being crunched.
Such was the story of Mesomeso-san as told by Marie's classmates. The girls find it to be scary, but Marie is entranced by this story, realizing that they were talking about her. In the library, Marie hears more students talking about the Mesomeso-san story, amazed at how much it spread. She remembers a day that Kanamori called her there, and she was left alone to cry for herself. She accidentally blurted out "I'm gonna crunch and kill you", and scared off a girl that was outside.
She thinks of how nice it is to become a youkai despite being a human when Kanamori suddenly appears, praising her for studying after school. He tries to assist her with her work and then tells her to meet at the usual place for "supplementary lessons." As soon as he leaves, Marie looks at the floor and begins panicking, not wanting to be a part of Kanamori's sessions any longer; as long as he has the videos, she can't do anything to him for fear of someone finding out. She then thinks about becoming Mesomeso-san and killing him when sees a girl's legs sitting in the seat in front of her. She looks up only to find nobody around her. She checks under the table again and finds nobody, thinking she's just being crazy when the same girl appears across from her and asks if she's Mesomeso-san.
Marie is bervous to see this strange person, noting how she looks just like a doll. Marie nervously gives her name, which the girl misinterprets as "Moriya Mesomeso-san." Marie corrects her, saying she's not Mesomeso-san and and the girl says it's a pity, as if she were Mesomeso-san, she could've been the eight mystery. Marie begins thinking about why there could be seven school mysteries, but the girl finishes her thought. Marie is surprised, and the girl explains that youkais swarm over people's feelings, and because children believe in youkais even in this era, a school would be the perfect place for youkais to gather. Among the youkai, the seven strongest are the seven mysteries of the school. Marie then asks why there's an 8th, and the girl says that the deactivation of the old school building caused the new school to become so big that there are more than seven seats available for the youkai; Mesomeso-san has been chosen to take the eight seat. The girl finds it a pity to see that Mesomeso-san is just a human, and Marie suddenly transforms into a puppet. The girl steps on Marie and torments her, citing her jealousy at Marie for stealing the hearts of so many children despite being human, pitying her even more for saying that she isn't Mesomeso-san. The girl determines that Marie must be punished and made into a doll, choking her. The girl finally introduces herself as Higanbana, the 3rd ranked of the seven mysteries and the dancing doll of the infirmary. Higanbana says goodbye as Marie realizes that she really was called the school's master. Marie thinks she's innocent, having never done anything herself and that it as the school who turned her into Mesomeso-san. Marie doesn't want to die, but begins to realize that as long as she's alive she'll keep suffering. Higanbana suddenly turns Marie back to normal, seeing that she now has the resolve to become a youkai.