In the autumn of 1998, a bizarre disappearance occurred in Room 409 of Beishan University's female dormitory. Four girls disappeared collectively in the middle of the night, leaving only a diary filled with strange rules. Twenty years later, when Lin Wei moved into the renovated Room 409, the regular knocks on the door at midnight, the unknown water stains seeping out of the walls, and the forever missing footage from the surveillance dragged her into a cycle forgotten by time. As the investigation deepens, she discovers that this dormitory hides a terrible sacrifice rule: every twenty years, the dormitory must "accept" a non-existent roommate, otherwise the entire floor will fall into disaster. And this year's deadline is approaching, and she is the last target...
