Se-hee is haunted by her step sister Tae-yeon who was considered a swimmer that is promising however murdered herself in a pool two decades ago. She unites the study camp for upcoming college entrance exam together with elite students at faculty. At the first night, when Se-hee finds a passage that is riddling scribbled in the desk, then she sees a dead girl's body wrapped upsidedown. A TV monitors are on and the students watch the other friend. Whenever they fail to answer the given questions punctually since that time, cruel killings' series occurs. The hidden facts behind Tae-yeon's suicide is revealed and before they end up being the next victim, students find it difficult to passing to undo the puzzle.