In this chapter, we learn that the d-note was stolen and that the next victim is "kaiyuan yang" . The narrator explains that the switch demon is not hiding its figure well, but that the first case on which it has been exposed exposes its place. There are two staircases in the school building, one near the entrance and the other on the other side of the building. Usually people go up the first staircase and then go down the other one. The first staircase is closed and the second staircase is open. When someone falls, everyone thinks that mumu is back and just runs. Yejie Ma who was in the front fell and tripped us. You idiot , I came up with a plan . So, mumujiang was actually the person because she co-conspiratored with the murderer. The murderer should have instructed her to play the prank of talking to you and then let you fall. 2 pretended that they were mumu and then spilled lubricating oil onto the stairs. The second staircase was open and mumu ran across the hall and joined the murderer on the fourth floor. While you experienced the series of accidents, the murderer gained enough time to commit the crime.