Recently I have watched many documentaries as well as movies about this kind of plot (DID), I also learn about DID, so here is the plot I understand after learning about DID.
Okay, first of all, please watch the PV first. Link here.
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We’ll start at the scene in which the inspector stepped out of the car to go to check the crime scene up (00:20).
And here is the crime scene, we can see the teacher’s strange face, all the girls are suspects (00:33).