For some reason, nailing down horror plots is harder for me than mystery plots, so this is even more tentative than usual.

  1. There is a problem.
  2. The backstory* is so epic that it dominates the present or real story.
  3. Wait wait this problem comes from the epic backstory that we never talk about and didn’t learn the real lesson from.
  4. WE MIGHT HAVE TO TALK ABOUT THIS, THE HORROR.
  5. If we talk about it, it might go okay; if not, kablooey.

This is a lot of Stephen King.  The ShiningBag of Bones, that kind of thing.  Nazareth Hill by Ramsey Campbell.  We Need to Talk About Kevin, by Lionel Shriver.

Mysteries have frame stories, too, that center around a crime in the past that was never resolved.  I see a lot of overlapping.  But I think the main thing that makes it horror here (plotwise) is that the thing Must Not Be Spoken About, where the mystery aspect seems higher if the thing is All Full of Dead Ends.

 

*Which might be plural.