Melancholia

Yoon Su poses a difficult question to students hoping to join the math club at Aseong High School, but no one answers correctly.

Yoon-su finds out that Seung-yoo solved a tricky math problem; Asung High organises International Mathematical Olympiad in order to select a student representative; Yoon-su puts Seung-yoo in detention.

Seung Yoo surprises everyone by excelling in the math competition and being selected to represent Aseong High School with Ye Rin; some parents and Ms Noh are unhappy with Yoon Su's teaching methods.

While Seung Yoo and Ye Rin prepare for the Math Olympics, Yoon Su uncovers corruption in the competition. She goes to Jeju Island to support Seung Yoo.

Seung-yoo confides in Yun-su about his past trauma; suspicions about Ye-rin's preferential treatment for the tournament begin to surface.

Seung-yoo confesses to Yoon-soo, which leaves her confused; Kyu-young attempts to blackmail Seung-yoo; Yoon-soo notices something odd in Ye-rin's midterm answer sheet.