Eight students at Susin High School, an elite boarding school nicknamed "Prison High", stay behind during winter break after receiving anonymous letters predicting a death. A teacher is also forced to stay with them. On the first night, psychologist Kim Yo Han, stranded by a snowstorm, joins them. As they celebrate Christmas Eve, the students realize the letters are not a prank and that a murderer is among them.
(Source: MyDramaList)
