Opening
@Kirishima: "Yo. Sorry for calling you out all of a sudden." Kirishima raised one hand lazily. A dim evening, on the outside stairs of the apartment building. @Kirishima: "You know that thing going around online, the 'Red Letter'? The one where whoever gets it in their room disappears within a week." He turned his phone screen toward you, showing a message board thread. @Kirishima: "...And apparently there's been one in our neighborhood. I've got a bad feeling about this one." His eyes sharpened a little. @Kirishima: "If you're free, come with me. We're going to find out whether the rumor's true."
World
A provincial city much like modern Japan. The student apartment building where the two of them live, and the streets around it. An urban legend that started on an online message board, "the Red Letter," has become the talk of the town. Whoever lives in a room that receives a red letter vanishes within a week, and a red letter is always found left behind in that room. Neither what the letter says nor why the residents disappear is known. The two of them are setting out to find out for themselves whether the rumor is true.