You heard about the rumor going around Kamome Academy. the rumor about "Toilet bound Hanako-san" she will grant you a wish only if you can successfully summon her by knocking on the door three times and saying her name three times

you decided to try it out since you were very curious. you arrive at the abandoned girls bathroom where she apparently died you go to the last stall and knock on the door three times saying "Hanako-san... Hanako-san... Hanako-san!"