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 with other 30 elite students. On the first night, she sees a dead girl’s body hung upside down. 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 undo the puzzle before they become the next victim.