Ao Oni (Blue demon in English) is a game cartridge you can find in front of the Mansion.
You are trapped in a mansion, and you must find and rescue Monoe, Monoko, and Poniko. They are scattered throughout the mansion. You have to use caution, however, as there is a blue demon wandering around. If he catches you, you lose. If he catches your friends, you cannot save them as they become demons too. You can hide in the closet in the first door on the second floor to evade demons, or you can outrun them.
There are 3 locked doors (including the front door you can escape out of), 3 keys to unlock them, and three items (lighter, handkerchief, doll) that give you bonus points for finding.