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 discovers that Seung Yoo solved a tricky math problem but he denies liking math; a math competition is held at Aseong High School to select a student for the International Mathematics Olympics.

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 shares a traumatic experience with Yoon Su, who offers comfort and understanding; Sung Jae witnesses the moment and feels overwhelmed; suspicions arise that Ye Rin got preferential treatment for the competition.

Yoon Su becomes emotionally conflicted when Seung Yoo confesses his love to her; Gyu Yeung seizes this opportunity to blackmail Seung Yoo by exploiting his feelings; Yoon Su discovers something suspicious in Ye Rin's test.