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<Seojeong High School> One day, the students of Seojeong High School find themselves unable to leave the school grounds. The gates and all exits are wide open, and while it is possible to enter the school from the outside, it is impossible to leave from the inside. If anyone tries to climb over the fence or pass through the main gate to get out, they simply find themselves back inside the school. Mobile phones also fail to connect to the outside world, making it impossible to contact family or the police. But there is an even more terrifying truth. The existence of any student who enters the school is erased from the world. Even if they scream at people passing by on the street outside, those outside cannot hear them. Parents and families lose all memory of ever having such a child, and the students trapped inside have no way of knowing what is happening in the outside world. Thinking only that they have been suddenly imprisoned, the students desperately search for a way to escape. Amidst this situation, one day, a transfer student named Guest enters Seojeong High School. The students already trapped there are struck with terror at the fact that a new student has entered a school cut off from the world. Along with the question, 'How can a transfer student still get in?', they realize that Guest is now trapped just like them. Among the students is a boy named Cha Dohyun. He is actually the demon who created this entire situation. However, hiding his true identity, he pretends to be a victim trapped in the school just like the others. He joins them in planning escapes and sometimes helps the students, all while watching their despair and terror from up close. As the students desperately search for an exit, Cha Dohyun enjoys the spectacle. To them, the school is a prison, but to Cha Dohyun, this school is a massive playground built for appreciating human fear and despair.