Ok Da-Jung is the youngest team leader in the cosmetics industry. She has divorced three times so far. She doesn’t care what others think about her and she also has quite a temper. Nam Jung-Gi works as a section chief at the same cosmetics company as Ok Da-Jung. Unlike her, Nam Jung-Gi has a timid and nice personality. He can’t say anything that makes others uncomfortable. He is able to make Ok Da-Jung’s blood boil.

Se-hee is haunted by her step sister Tae-yeon who used to be a promising swimmer but suddenly killed herself in a swimming pool 2 years ago. During the summer vacation, she joins the study camp at school for upcoming college entrance exam with other 30 elite students. On the first night, when Se-hee finds a riddling passage scribbled in the desk, she soon sees a dead girl’s body hung upside down. Then the TV monitors are on and the students watch another friend crushed by his bike in the corridor. Since then, the succession of cruel killings occurs whenever they fail to answer the given questions in time. Meanwhile, the hidden truth behind Tae-yeon’s suicide is slowly revealed and terrified students struggle to death to undo the puzzle before they become the next victim.

On the eve of the dreaded university entrance examinations, the lives of 24 of the smartest and brightest students at one high school, are on the line. Locked inside a classroom and nowhere to escape, a television monitor broadcasts a terrified Hae-yeong, the top student at the high school, tied up and seated on a chair inside an aquarium that slowly begins to fill with water. A voice from the monitor announces if the 24 cannot finish the exam administered within the allotted time given, Hae-yeong will die and so will everyone held captive inside the classroom. Thus begins the examination that will determine whether they live or die.