There exists a ghost story, the second of the Seven Mysteries of Seikyou Academy. Called "Hidden Demon", it consists of a game that is in fact a certain "ceremony".
The player shall prepare a doll with a name attached to it and after the final bell has rung, shall begin the game with the words "Let's play 'Hidden Demon'. I'll be the demon, okay?" twice. A number is chosen by the player, and he or she counts back from ten to one. Then, by stabbing the doll, the player declares his or her victory.
Now, the owner of the name is now the demon, and the player must now hide the number he or she chose in a location in the same floor where the game began; a waiting game soon follows.
There are three "simple" rules to the hiding part of the game: one, the player must not leave the building; two, the player must not say a single word; and three, the player must not be "found". Although the game is supposed to be played alone, there is a certainty that someone will be able to find the player in the hiding part; when this happens, the player can opt to end the game by telling so to the doll.
However, when the doll is nowhere to be found, the "Hidden Demon" will be summoned to hunt down the player.
Teiichi recently discovered the story behind the Second Mystery of Seikyou Academy and explains it to Yuuko, also adding the detail that it was Yuuko's name attached to the doll. This day is supposed to be the start of the Paranormal Investigation Club's activities, and in spite of Teiichi's detemination to solve Yuuko's death, Yuuko doesn't seem interested to dwell in serious talk.
As Yuuko begins to become clingy to Teiichi, the Paranormal Investigation Club is interrupted by the presence of a frightened girl.
A girl who is afraid that Yuuko is going to be killed soon.
Of course, both Teiichi and Yuuko are surprised at the girl's unexpected warning, especially Yuuko herself, who thinks that she — being already dead — can't possibly be killed.
After a short interview by Teiichi, he finds out that the girl, named Momoe Okonogi, played "Hidden Demon" three days ago, and she was afraid for her life after the doll she used suddenly disappeared. Also, Momoe is concerned about the well-being of the owner of the name she stabbed in the "Hidden Demon" game, so she pleaded for Teiichi to investigate the incident.
Teiichi admits to Yuuko that he does not feel confident in handling such a case, so he decides to ask help from Yuuko, who acted as the club's "ghost president" to avoid any suspicions in Momoe's part.
With a girl with zero confidence as a client, Yuuko decides to just hand over a hand-made amulet to ease her worries and she leaves more relaxed than before. Meanwhile, Teiichi notices how Yuuko treated Momoe, and Yuuko points out that bad things happen to people who play strange games after school.
As Yuuko tries to explain what might have happened with the doll, Teiichi notices a stuffed bunny doll with a knife sticking out of it. To his shock, he discovers that it might be the same doll that Momoe was looking for, and Yuuko ended up ruining the game. Teiichi soon suspects that she sabotaged the game, knowing that Yuuko knows that story of the Hidden Demon, but she denies the accusation.
In fact, she picked up the doll because it bore her name on it, and she felt hurt in seeing someone stab her name, oblivious to the implications of the things she did.
Yuuko soon finally decides to help out in Momoe's case, but only after clarifying something important: the things a player feels when playing "Hidden Demon" is merely self-suggestion, thus everything they think they perceive is in fact a figment of their own imagination. However, such an illusion will be difficult to dispel unless they do something for the person to forget such a thing exists.
Teiichi soon discovers that the only way to cover up the lie Momoe created is to douse it with a bigger lie, and that is what Yuuko has in plan.
Inside the old building, Momoe is still holding on to the charm that Teiichi gave to her. On her way, though, her grip slips, and as she tries to pick up the charm, a ghastly apparition appears behind her. Momoe felt helpless for a moment until Teiichi pulls her away from the apparition. As the apparition pursues them, Teiichi explains that they needed to exterminate the one chasing them to end the "Hidden Demon" game, and he has a genuine amulet prepared for it.
Soon, the ghostly figure emerges behind them and Teiichi uses the amulet to strike it at the chest; a piercing cry is heard as the figure fizzles away, leaving the doll that Momoe lost. Teiichi quickly gives the doll to Momoe and watches as she gives a short word of thanks before leaving.
The performance is finished.
Yuuko emerges from the nearby classroom dressed in a traditional Japanese ghost costume and asks for a comment on her performance. Teiichi expresses his surprise upon seeing Yuuko in a frightening form; Yuuko explains it as Momoe's perception of Yuuko's presence... something that saddened Yuuko for a moment.
Also, Teiichi notices that they fooled another person with a fake. It is there that Teiichi remembered touching Yuuko's chest while performing the exorcism, and Yuuko uses the chance to tease Teiichi.
In the end, Yuuko still does not understand how she ended up being related to the "Hidden Demon" game.
Soon, Teiichi receives a visit from Momoe in the Paranormal Investigation Club, and she personally thanked him for exterminating the spirit that was out to get her. As Teiichi is praised by Momoe, Yuuko tries to make her presence felt as she feels that she needed to be credited for her role as well.
(in order of appearance)
- "You're pretty cold."
"Obviously, since I'm already dead." — Teiichi's reaction to Yuuko's cold treatment towards Momoe, and her response