Melancholia

At Asung High, Yoon Su’s new workplace, she gives a tricky question for students who want to join the math club. While no one in the class got the question right, a correct answer sheet is posted on the board outside the classroom. To find out who did it, Yoon Su provokes them and manages to find who it is.

Yoon Su finds out that Seung Yoo solved the question. But he tries to keep a distance from her, saying he doesn’t like math. Yoon Su gives him another math problem, and the problem allows them to connect and communicate. Meanwhile, a math competition is held at Asung High to select the rep student to participate in the International Mathematics Olympics.

Seung Yoo participates in the math competition and is picked to represent Asung High along with Ye Rin. The result shocks everyone who thought Seung Yoo had no talent in math except for Ye Rin who knows that Seung Yoo is a math genius. Meanwhile, the parents of other students and Ms. Noh aren’t happy with how Yoon Su runs her math class and club.

Seung Yoo and Ye Rin prepare in earnest for the Math Olympics. Jung Ah and Min Jun teach Ye Rin the means to win, and Yoon Su finds out about the corruption in the competition. However, Yoon Su goes to Jeju Island with Seung Yoo to help him with the contest. On the same day, Ye Rin arrives at Jeju Island for internship activities for the competition.

After the Math Olympics, Seung Yoo opens up and tells Yoon Su the traumatic experience he had bottled up inside of him for a long time. Yoon Su comforts him and sympathizes with Seung Yoo's pain. However, Sung Jae witnesses the moment and gets overwhelmed with a strange feeling. Meanwhile, suspicions that Ye Rin got preferential consideration for the competition to become an issue.

Yoon Su gets overwhelmed with mixed and complex emotions after hearing Seung Yoo profess his love for her. Gyu Yeung notices Seung Yoo's love for Yoon Su and uses it to blackmail him. Meanwhile, Yu Hye Mi takes things into her hands for the midterms Ye Rin is about to face. However, Yoon Su finds something suspicious in Ye Rin's midterm answer sheet.

After four years, Seung Yoo has now become a renowned mathematician, and he returns to look for traces of Yoon Su who has disappeared after the incident in 2017. Meanwhile, Principal Noh Jung Ah of Asung High School of Math and Arts proposes an offer to Seung Yoo, and Seung Yoo responds with another offer.

Now that he has reunited with Yoon Su, Seung Yoo tries to get close to her again. Noh Jung Ah, who doesn't know that Yoon Su has become an elite instructor at an academy, asks the academy for the guidance of her daughter, Ji Na. Meanwhile, Sung Jae appears in front of Seung Yoo and Yoon Su, who are together.