Subete Wasurete Shimau kara

Mystery writer "M" has a girlfriend of five years named "F". After a trivial tiff on the night of Halloween, "F" goes off the radar for a good three weeks. At the suggestions of those around him, "M" starts to search for "F" but gets a shock to find out from those around her that "F" is actually different from what he has always known.
(Source: Doramaworld)
~~ Adapted from the essay "Subete Wasurete Shimau kara" (すべて忘れてしまうから) by Cinder (燃え殻).