In 2009, Bok Su, who is very popular among his school friends but bad at studying, and Soo Jeong, who is known to be the prettiest and smartest girl in Seolsong High School, get to team up and to improve Bok Su's grades. Soo Jeong's private lesson makes Bok Su interested in studying, and Bok Su realizes that his feelings for Soo Jeong have started to grow.

In 2018, Soo Jeong gets conned by her colleague and ends up paying a great deal of money to become a full-time teacher at Seolsong High School. At the same time, her student named Young Min suddenly disappears after leaving a post on his social media page about the overly competitive system that exists in his school.

Bok Su dives into the river and saves the woman who seems to have tried to kill herself. Then, he gets shocked by the fact that the woman was Soo Jeong whom he liked in high school. In the meantime, Young Min's revelation concerning Sulsong High School's education system stirs up the public outrage, and even a group of people starts to protest in front of the school.

Mr. Park who was Bok Su's homeroom teacher in high school comes to visit Bok Su and suggests that he should go back to school and finish the study. Mr. Park's suggestion makes Bok Su recall a painful memory where he was falsely accused and forced to drop out of the school. In the meantime, Se Ho, who used to be Bok Su's schoolmate, comes back home from aboard.

Bok Su puts his old school uniform back on and marches into his old school he dropped out almost a decade ago. But the start doesn't seem easy. Old time friends with complicated history show up to be his homeroom teacher and the new chairman of the school board. Just to make things worse, Bok Su is assigned to the same classroom as his nephew, and witnesses In Ho being bullied to his classmates.

After spending his first day back to school in utter shock, Bok Su tosses and turns throughout the night thinking of Soo Jeong and about what happened between them in the past. Soo Jeong is also sleepless thinking of Bok Su's unexpected comeback as her student. The next day, Bok Su calls Soo Jeong to follow him up to the school rooftop to wind back time and resolve what happened in the old days.

Bok Su luckily made it back to school but realizes that he needs to come up with a revenge plan. A plan to get back at Se Ho and Soo Jeong. He tries stalking Se Ho and sneaks into his chairman office but fails. Next, he goes to the Administration Office when everyone is out during lunchtime. Right when he was trying to scan through some papers, a surprising guest makes an entrance.

Young Min, the student with the worst grades in Ivy class is suggested to leave his glass booth and degrade into SKY class. Trembling with anxiety in feeling like he's being kicked out of a place he needs to be, Young Min locks himself inside of his glass booth. Everyone is under panic, trying to persuade him to come out. Right then, Bok Su shows up with a chair and runs up to break one of Sulsong High School's most symbolic system.

Soo Jeong says she will take full responsibility for Bok Su. Bok Su feels uncomfortable about it but at the same time, he is worried about Soo Jeong. Meanwhile, Young Min transfers his class to Wildflower Class.

Soo Jeong looks for Seung Woo as he keeps skipping school. Soo Jeong finally finds where Seung Woo is, but gets into a fight. Bok Su gets a wound while protecting Soo Jeong. Meanwhile, Bok Su gets angry after seeing Soo Jeong and Se Ho together.

Se Ho decides to offer financial support to the students in Soo Jeong's class despite the head master's disagreement. In the meantime, Soo Jeong receives a message from Kim Hyung Ho. He says he can pay back 50,000 dollars to Soo Jeong. However, Bok Su finds out that Se Ho is behind Kim Hyung Ho.

Bok Su tries to talk about what really happened in the past and to get rid of misunderstandings with Soo Jeong, but she doesn't even try to listen to him. From the next day, Bok Su starts not to go to school and even thinks about quitting again. Soo Jeong is worried if his absence might affect her career.

By talking about what exactly happened to each other nine years ago, Bok Su and Soo Jeong clear up the misunderstanding between them. Bok Su comes back to school and brings beverages and food to Soo Jeong. Soo Jeong is also happy, but she tries to hide her feelings for Bok Su just because she's his teacher.

Mr. Park sends Bok Su to Chungju so that he can accidentally meet and spend a day with Soo Jeong. Bok Su and Soo Jeong tour the city for the site survey, and they ride a bicycle recollecting old memories from nine years ago. After they finish the site survey, Bok Su tells Soo Jeong his feelings for her.

In Ho has been lying to his mother that he was in the Ivy Class, the smartest class in Sulsong High School. One day, Ms. Kang visits Sulsong High School feeling like she should support him as a parent. However, she finds out that In Ho has been forging his report cards and awards when his grades were poor, and that he was in the Wildflower class. Completely in shock, she causes a ruckus and leaves telling In Ho to never come home.

Rekindling with Bok Su, Soo Jeong and Bok Su makes a huge decision to get together again as a couple. Noticing her and Bok Su’s relationship, Se Ho calls Soo Jeong aside to share his heart. Soo Jeong tells Se Ho to stop relating himself to her and admits her new relationship with Bok Su. Se Ho makes a smirk and tells Soo Jeong to come to him whenever she wants. But that it’s going to be tougher for her and Bok Su from now on.

Bok Su and Soo Jeong promise to each other that they should overcome difficulties ahead of them. Bok Su officially introduces Soo Jeong as his girlfriend to Min Ji and Gyeong Hyeon. Meanwhile, the school is planning a debating contest. From Wildflower Class, Young Min, Seung Woo, and Bok Su are chosen to participate in the contest.

Before the debating contest is held, Bok Su finds out about the connection between the school and a private educational institute and that the institute sells the topic to the students in Ivy Class. Bok Su sneaks into Ivy Class students' glass booth to get the debate topic in his hand. And the debating contest is held as scheduled.

Seung Woo accidentally finds out that Bok Su and Soo Jeong are dating, and Bok Su begs Seung Woo for keeping it secret. Meanwhile, Chae Min and Young Min are in a fight at school, but only Young Min is subjected to punishment. Soon after, Soo Jeong gets a video as the evidence which can prove that Chae Min was the one who started the fight, and Bok Su plans to reveal the truth.

Min Ji finds out the awards have been manipulated and Dong Jun submits an indictment to the Education Office about the corruption happening in school. However, Se Gyeong steps in and stops the investigation, and she demands that Se Ho should get out of her school.

For submitting an indictment to the Education Office about the corruption happening in school, Dong Jun gets suspended. In the meantime, Bok Su tells Se Ho that the students in Wildflower Class won't be transferred if they make their class average grades. And Bok Su persuades his classmates into focusing on study.

Dong Jun starts to protest alone in front of the school gate for the fact that the school has been discriminating students and committed corruption. Meanwhile, Bok Su and Gyeong Hyeon secretly gather evidence that will prove Se Ho's wrongdoings, and they plan to leak the proof to the media at the right time.

Bok Su collects evidence of the school's wrongdoings and starts investigating the corruption. Bok Su even collects evidence by using a false identity and stealing the file from a parent's phone. Meanwhile, Chae Min fails to receive the highest score even after asking for the re-test.

Soo Jeong tells Se Ho not to remove her name from the list of employment solicitation, and tells Bok Su that she will stand as a witness. Se Ho threatens Soo Jeong once again with pictures of Soo Jeong and Bok Su and after seeing them, Soo Jeong decides to break up with Bok Su for a while.

Bok Su and his friends get the evidence that the test questions have been leaked for Chae Min. And Soo Jeong decides to reveal that she handed money to get a permanent position. Also, Soo Jeong casts a bait to catch Chae Min and to give him one last chance of telling the truth about the leak of test questions and answers.

As Bok Su and his friends planned, Bok Su reveals all the irregularities that the school has committed through an online broadcast. Soo Jeong makes a confession as well. After the disclosure, Bok Su and Soo Jeong happen to overhear a conversation between Se Ho and his mother, Chairwoman Lim, after a press conference.

Se Gyeong gets photos of Bok Su and Soo Jeong being together. Using this, Se Gyeong threatens Soo Jeong to not testify. Soo Jeong hands in her resignation letter to still testify against the school.

Bok Su and Soo Jeong go on a trip to the ocean together to make up for their unfulfilled wishes. While on the trip, Soo Jeong asks Bok Su to get back together and love each other once again. Together they go and testify against the school to disclose the truth.

After realizing what he has done is wrong, Se Ho decides to help Bok Su reveal the truth behind Sulsong High School's corruption. Bok Su doesn't trust Se Ho and thinks this is another trap set up by him. Even so, Se Ho tries his best to make up for what has happened nine years ago by disclosing the truth to the public.

After a successful hearing, Bok Su and his friends celebrate their victory. Soo Jeong has quit her job to atone for her wrongdoings. Se Gyeong, on the other hand, shows no sign of remorse or regret and is closing the school down.

With the help of Bok Su, Se Ho testifies against his mother. The scene of Chairwoman Lim's arrest is aired live, and the people enraged by the situation sign the national petition.After the position becomes vacant, Bok Su asks Mr. Park to become the principal of Sulsong High.

The graduation ceremony begins. Bok Su delivers a speech in front of other students. In 2023, Bok Su comes back to Sulsong High School again and Soo Jeong still tries her best to become a certified teacher. Meanwhile, Se Ho visits her mother who has just been discharged from jail.