Synopsis
In 2009, Kang Bok-soo is enrolled as a junior at Seolsong High, a prestigious private high school known for its discriminatory practice of segregating students based on their grades and social status. Bok-soo is a routine troublemaker who scores poorly in tests, but he is also popular for being a bully hunter. He dates the school's smartest student, Son Soo-jung, and is also best friends with Oh Se-ho, the awkward son of the school's abusive board chairwoman, Lim Se-gyeong. Soo-jung has long been hiding the fact that she is poor, but her background is suddenly outed to the school a day after she discloses it to Bok-soo, causing her to believe that he cares for her out of simple pity. When she heads to the roof to mourn, she hears Bok-soo and Se-ho having an argument, ending with the latter falling. Se-ho survives the incident and testifies that Bok-soo pushed him. Soo-jung, still heartbroken by Bok-soo's apparent betrayal, testifies against him as well, leading to his expulsion from Seolsong.
Nine years later, Bok-soo is working at Your Request, a general service company. Soo-jung is struggling to receive a promotion as permanent teacher at Seolsong and unwittingly scammed to pay a hefty sum of money to become one. She is currently counseling Oh Young-min, a troubled student of Ivy, the school's highest-ranked class. She talks down Young-min when he attempts to commit suicide by jumping into the Han River, only to end up falling down with him. Bok-soo, who happens to be standing nearby, saves her. His heroic action forces Se-ho, recently promoted to become Seolsong's board chairman, to readmit Bok-soo into the school and appease public opinion. Bok-soo enters Wildflower, the school's lowest-ranked class. Se-ho pulls some strings to promote Soo-jung as permanent teacher and gives her the job to mentor Wildflower.
Bok-soo devises a plan to take revenge against Seolsong, and Se-ho in particular, for the lies that caused his expulsion. Though initially chilly, Bok-soo rekindles his relationship with Soo-jung when they learn that their grudge was driven by a series of misunderstandings: Se-ho was the one who gossiped Soo-jung's background and framed Bok-soo into playing the part of his rooftop fall, which was actually a failed suicide attempt. Se-ho has long been in love with Soo-jung and framed Bok-soo precisely so they would split. When Soo-jung confronts him about this, he openly admits his actions and warns her that, for rejecting him, he would make her and Bok-soo's lives miserable. Aided by his friends, Bok-soo slowly dismantles the corrupt atmosphere of Seolsong. In the process, they have to face the school's entire upper management, all of whom want to preserve the status quo. Upon collecting enough evidences, Bok-soo uploads a YouTube video containing documents that implicate Seolsong's corruption.
The ensuing scrutiny convinces Se-gyeong to depose her son and engineer a scapegoat so the school's reputation could be protected. Se-ho, having had a change of heart and sick of being mistreated by his mother, forms an alliance with Bok-soo to further scrutinize Seolsong and bring them to court. Bok-soo publicizes Se-gyeong's plan to turn Seolsong into a corporation, and Soo-jung further testifies against her at the cost of her job. Se-ho, meanwhile, admits the truth about his suicide attempt. Not willing to surrender, Se-gyeong blackmails Se-ho into becoming a scapegoat and plans to close Seolsong. Bok-soo, Soo-jung, their friends, and the teachers band together to prevent her from entering the school long enough until the police arrive to arrest her, having been tipped by Se-ho regarding his mother's crimes.
In the aftermath of Se-gyeong's arrest, Seolsong becomes a public school. Se-ho moves to the United States to recover, while Bok-soo receives his high school diploma a year later. Four years later, Bok-soo is teaching at Seolsong, while Soo-jung finally becomes a teacher through legitimate means.
