It's basically the same as that of the manga's first "episode." Surprise, surprise.
It follows the same chain of events that I'm familiar with after reading the whole manga. Here's how it goes, based on each manga chapter (the raws for its remaining chapters are available):
1. A girl named Marie Moriya is harassed by her classmates and teacher Kanamori, who is most likely molesting her (I know he does in the manga, but they don't show it in the VN, so I don't know if it's just mentioned or implied).
2. Marie accidentally starts a rumor of "Mesomeso-san," a youkai who haunts the toilet stall in an abandoned building near the school where Kanamori often "meets" with her. She is approached by Higanbana (one of the seven real youkai who haunt the school), who decides to make Marie a real youkai.
3. Kanamori becomes afraid that the rumors of "Mesomeso-san" will get him exposed. Marie goes to the toilet stall where she confronts Kanamori over their "meetings" while she's still human.
4. Kanamori freaks out and strangles Marie to death, then dumps her body in a sewer.
5. Kanamori declares himself to be the real "Mesomeso-san." Higanbana tells Marie's ghost that there will be a poll between Marie and Kanamori to settle which one of them gets to be "Mesomeso-san."
6. Kanamori returns to the toilet to dispose of evidence implicating his murder of Marie (a button that fell from her shirt). Marie's ghost tries to haunt Kanamori.
7. Kanamori somehow gains emotional and psychological leverage over Marie's ghost and "kills" her again, seemingly winning the poll.
8. Higanbana shows up and tells Kanamori that he's done something that cost him his chance to beat Marie (I'm not sure what). Marie becomes a youkai, "thanks" Kanamori for something (again, I don't know what), and eats the sunnuvabitch. She and Higanbana become buddies, or something along those lines. The end.