Plot

Students at an elite high school are held captive and forced into a series of sadistic games. They are picked off one by one and held in impenetrable traps where they must rely on the amazing intellects of their classmates to be released.

At an exclusive high school in Seoul, examination marks matter and so do parents' social status. To prepare for a visit from students from England, two popular teachers, Mr. Kim and Ms. Choi, gather top pupils for a Saturday study session. Things go awry: one by one, students disappear, dying in grisly ways, sometimes in view of the others via closed circuit TV. A voice on the P.A. system warns of death if anyone tries to leave. The voice also asks questions, and Mr. Kim leads the students in trying to solve them, hoping to save a student. Will the ordeal come to an end, and is the cause rational or supernatural? What's the pattern?—<jhailey@hotmail.com>

A group of 20 high school students partake in an elite class to prepare for a college exam. Among them are the rebellious Kang I-na, her best friend Yoon Myong-heo, and the class clown Kang Hyun, who harbors an affection for I-na. A few days after incidents involving a student, Beom sees a ghost haunt his paper and class teacher Hwang Chang-wook discovers scars on his hand. The class is interrupted by a TV showing the top-ranking student, Min Hye-yeong, trapped inside a glass tank that slowly starts to fill with water. A voice over the PA system announces they are part of a death game with questions that must be solved by the class; failure to answer them in time will lead to the student's death. It also warns them and their two teachers, Hwang Chang-wook and Choi So-yeong, against leaving the school. The class is asked to solve the equation that is written on the glass tank that their classmate is trapped in, but fail to solve the first question to save Hye-yeong who subsequently drowns.

Despite the warning, the class tries to leave, only to find that a teacher and hall monitor, Mr. Lee, was beaten to death, leaving only security guard Mr. Kim to protect them. The students split up and try to answer the questions while also attempting to find a way out. Several students die in brutal death games set up by a mysterious woman as the class solves all the puzzles too late, which include one of their classmates who has wax dripped on him slowly and fail to save him as the wax eventually suffocated him. However, they later notice that each puzzle contains a clue pertaining to a student, Kim Ji-won who died earlier that year under mysterious circumstances though it was said to be a suicide. I-na discovers students are dying according to their ranking on the last test. Beom, who earlier tried to strangle I-na and was put in a mental facility, escapes and attempts to kill I-na, but Hyun shields her from the attack and dies. Chang-wook then kills Beom. I-na discovers to her horror that Myong-heo has also been kidnapped by the woman. They succeed in solving the trap but the killer still kills her by dropping her from the roof, and Chang-wook climbs the roof and strangles the woman to death.

A memorial is held for the students killed in the school's gymnasium, then I-na is kidnapped unnoticed by anyone. When suddenly over the PA system one final question is posed: whoever killed his daughter must step forward and confess. The person in question, the school counselor and a now-demented Chang-wook, reveals himself and is killed by the mastermind, who turns out to be Kim the school security guard. I-na is then freed by So-yeong.

Flashbacks reveal that Kim's daughter, Kim Ji-won, worked hard to enter the elite class. However, the parents of the killed students bribed Chang-wook for the entrance test answers, giving them an unfair advantage. After being disqualified and finding out about the plot, Ji-won confronted Chang-wook and he kills her to cover up his misconduct. Since Ji-won's death, the life of her parents fell apart as the family home was seized by loan sharks. Out of rage, he murdered the loan sharks and along with his wife, planned their revenge. The sole witness to the incident, Beom, slowly became mad as he felt that Ji-won had been haunting him ever since. Kim and his wife (the woman who hung Myong-heo) learn this from the cellphone of their daughter, who had been recording while being strangled, and decided to kill Chang-wook and all the students whose parents they deemed responsible for their daughter's death.

The film closes with I-na seeing that she topped the class ranking, and it is implied that Beom's attacks were due to her being possessed by Ji-won since the beginning as she is briefly shown as Ji-won.