Kim Yeon-hong (Son Ye-jin)appears to have a perfect life. She is beautiful, is married to a charismatic young politician (Kim Joo-hyuk), and her adolescent daughter who used to be a troublesome child is now doing better at school. Her husband, Kim Jong-chan decides to run for office against the powerful incumbent, No Jae-soon (Kim Eui-sung). A few days before the elections, Yeon-hong and Jong-chan's daughter, Kim Min-jin (Shin Ji-hoon) goes missing. At first, they assume she has run off to seek their attention as she had done before. However, Yeon-hong soon finds out that the friend Min-jin said she was going to meet does not exist. Then Yeon-hong learns from the school her daughter attends that she had an eccentric friend named Choi Mi-ok (Kim So-hee [ko]) and they were both bullied by their classmates.
Junior high school teacher Yuko Moriguchi (Takako Matsu) announces to her class that she will resign before spring break. Moriguchi reveals that because the HIV-positive biological father of her daughter Manami was ill, she used to bring Manami (Mana Ashida) to school with her. One day, after school, she returned to the room where Manami was but found her gone. Her daughter was later found drowned in a school swimming pool. She then explains that two pupils in her class, whom she dubs "Student A" and "Student B", have murdered her four-year-old daughter.
