Baekyeon Girls’ High School goes on a school retreat. While practicing diving, someone pushes Ha Rin into the water, and Ja Yeon jumps in to save her. The next day, Class Five plays a survival game using paint guns. The last five standing, Soo Ji, Ja Eun, Jae Hyung, Eun Jung, and Ji Ae are put in danger of ending up in Grade F.
Soo Ji moves up to Grade C, and the treatment she gets changed completely overnight. She breaks the promise she made with Ja Eun, but she decides to join hands with her and help the new girl in Grade F, Pyo Ji Ae. Soo Ji, Ja Eun, and Ji Ae come up with a plan to destroy the pyramid altogether.
Soo Ji is determined to figure out the characteristics of her classmates and the mastermind behind the Pyramid Game. She also tries to join forces with other girls who seem vulnerable. She even asks her homeroom teacher for help, only to be disappointed. Meanwhile, the 14th Pyramid Game begins, and Ja Eun, who kept skipping school, shows up.
Soo Ji transfers to Baekyeon Girls’ High School and discovers that her class plays a game called the Pyramid Game in which the students decide on a hierarchy based on a popular vote. As Soo Ji ends up in the lowest rank, she devises a plan to get back on her feet.
Every last Thursday of the month at an all-girls high school, the students of Class 2–5 cast their votes in a popularity poll. The results classify them into Grades A, B, C, D, and F. If they fall into Grade F, they become legitimate victims of school violence.