The plot follows a class of high school students attempting to overcome the stress of being ranked by their exam grades, and facing the difficulties of being a teenager in a high-pressure, corrupt system. Its central protagonist is Ra Eun-ho (Kim Se-jeong), a cheerful and kind-hearted 18-year-old who dreams of being a webtoon artist but is caught up in the search for a mysterious troublemaker in the school, known as 'Student X'. When she is accused of being Student X, her dream of going to university to study art is put at risk as she faces expulsion. Hyun Tae Woon (Kim Jung Hyun) is the son of the director of the school who enjoys his youth but hides deep scars. Song Dae Hwi (Jang Dong Yoon) is a bright student who's ranked 1st everytime but doesnt have enough money for his dream college.
The rebellious son of the school's director. He doesn't take school seriously and resents his father. He was involved in a motorcycle accident in which his best friend Joon-ki died after saving Eun-ho from the wreckage of a bus. He has crush on Eun-ho and he is beign X.
The school's student president. He is a seemingly perfect student who is always ranked first place, despite coming from a poor family. He is in a relationship with Hong Nam-joo, who comes from a poor family but appears wealthy. He was best friends with Tae-woon and Joon-ki before the accident. However after the death of Joon-ki, he and Tae-woon fell into a conflicted relationship.
Form teacher for Class 2-1, he cares deeply for his students and always believes in them against the advice of the other staff. He struggles to speak up against authority. He falls in love with the police officer Han Soo-ji.
A police detective who gets demoted for mishandling a case, and ends up assigned to the high school as the school police officer. She is an ace detective and a stickler for rules. She falls in love with teacher Shim Kang-myung.
Dae-hwi's girlfriend. She pretends to be from a rich family because she is ashamed of her father's actual profession as a taxi driver. She is frustrated and stressed because of her family's financial struggles.
Eun-ho's loyal best friend, who is a big fan of Issue. Her mother is a cleaner at the school and has given up her dreams to support her mother. She often gets into fights with Eun-ho but always makes it up.
Son of a prominent prosecutor. He is tutored by Dae-hwi but is jealous of him because he always comes in second place to him. When he was in grade 10 he used to date Seo Bora, but ends up splitting because he is obsessive and abusive. He really wants to prove who X is.
Hong Kyung as Won Byung-goo
Tae-woon's clueless friend, often seen drinking strawberry milk.
A target of bullying in the class. She is afraid to report Young-gun and her other bullies to teachers because of their previous failings. She used to date Kim Hee-chan in year 10 before she broke up with him for being abusive. She takes a while to trust people, and believes in forgiveness. She is a loyal friend to Eun-ho.
A troublemaker who bullies Bo-ra. She later trains to become a police officer, inspired by Han Soo-ji. She is loyal and will owe up to anyone's mistakes. She is conflicted with finding a place to belong.
In May 2017, KBS offered the leading role to Kim Yoo-jung as a follow-up project to her popular series Love in the Moonlight (2016). By June 2017, she officially declined. The first script reading of the series took place June 19, 2017 at the KBS Annex Building in Yeuido, Seoul, South Korea.