The Prison Prep School Murder Case File 2 - S1-E11
Stupidity: The whole story of the 5 episode arc requires unhealthy amounts of suspension of disbelief; the killer directly challenged the police and announced where and when the murders will take place; at a 3 days prep school camp. Despite that, one of the teachers, a new one at the job even, is allowed to attend the camp wearing a ski mask, supposedly to hide hideous burns, and the police just lets him in without checking. And yes, he's the criminal, not even a red herring. By comparison, in the live version adaptation they check him, and he's wearing prosthetic scars.
The Prison Prep School Murder Case File 2 - S1-E11
Stupidity: Akechi finds the pieces of chalk but does not investigate the open locker, where the victim left what turns out to be literally the name of the murderer. The plan of the all-knowing Puppeteer works only because the smart investigator does not perform even the most basic crime scene.
