Rumors start to spread at the catholic girls' high school afterwards Unjoo suicide . The younger sister Jung-un who attends the school of unjoo gets suspicious about Unjoo's death. After continual evaluation, Jung-un discovers that on the night of the incident, after making a vow on 40, Soy, Eugene, Eun-young the four of them had tried to commit a suicide. However, the three are still alive, and just Unjoo had died. The appetite, jealousy, also lies supporting the pledge of death between the four girls are revealed, together with the hidden truth behind the suicide of Unjoo.