Ji Yoon-soo, math teacher, set to marry, assigned to prestigious Asung High School in Gangnam.
Yoon-Soo realizes that Seung-Yu is the one who guessed the math club's Calculus question.
When Yerin learns that Seung-yu used to be a math prodigy, she feels uneasy.
Seung-Yu and Yerin start preparing for the Math Olympics in earnest.
Seung-Yu opens up to Yoon-Soo about a past trauma she hasn't been able to tell anyone about.
Yoon-soo has mixed feelings about Seung-yoo's confession.
Im Soo-jung
Ji Yoon-su
Lee Dohyun
Baek Seung-woo
Jin Kyeong
Noh Jeong-ah
Choi Dae-hoon
Ryu Sungjae
Oh Gwang-rok
Ji Hyunwook