Divorced mother Yoo Rim (Yoo Seon) lives with her teenage daughter Eun Ah (Nam Bo-Ra), who is having a tough time at her new school. She has a crush on classmate Jo Han (U-Kiss’s Shin Dong Ho), but he turns out to be different than what she expected. He and his friends rape her and then threaten her with a videotape of the assault. Eun Ah eventually takes her own life, leaving Yoo Rim alone and devastated. As minors, Jo Han and his friends are let off with light punishments. Yoo Rim takes matters into her own hand to get revenge for her daughter.

Na-mi, a transfer student, becomes a laughingstock on her first day of school because of her strong southern dialect. Six girls jump in to help the new kid adapt to the new environment, and the girls come to call their circle of friends “Sunny.”

Kyung-chul is a dangerous psychopath who kills for pleasure. He has committed infernal serial murders in diabolic ways that one cannot even imagine and his victims range from young women to even children. The police have chased him for a long time, but were unable to catch him. One day, Joo-yeon, daughter of a retired police chief becomes his prey and is found dead in a horrific state. Her fiance Soo-hyun, a top secret agent, decides to track down the murderer himself. He promises himself that he will do everything in his power to take bloody vengeance against the killer, even if it means that he must become a monster himself to get this monstrous and inhumane killer.

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.