The story of the Hidden Demon (隠れ鬼, Kakure Oni) is a ghost story which revolves around a one-player game that challenges the player and the spirit it is rumored to summon. It belongs to the Seven Mysteries of Seikyou Academy.
After school, more specifically after the last school bell for the day rings, a single student prepares a doll with a name attached to it and a cutter. The game begins when the player says "Let's play 'Hidden Demon'. I'll be the demon, okay?" twice, and the player must choose any number and count down from that number to one. Afterwards, the player claims his/her victory by stabbing the doll and the name with the cutter.
The second phase of the game then begins after the player stabs the doll. At this stage, the owner of the name is now the demon, and player must now hide the number he/she chose in a location at the same floor where the game started.
Three "simple" rules are to be followed at the second stage of the game:
- The player must not leave the building;
- The player must not say a single word; and
- The player must not be "found".
In spite of the fact that the game is supposed to be played alone, there is a certainty that someone will be able to find the player in the second stage of the game; 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.