TTeaching mathematics at the prestigious Ahseong High School, Ji Yoon Su (Im Soo Jung) loves her job almost as much as she loves math. A good-natured woman, with a tenacity that is always driving her forward, Yoon Su is an excellent teacher who loves to share her fondness for math with her students. Extremely passionate about the subject, Yoon Su is always encouraging her students to find their own answers and is happy to come alongside those who might need a little extra help. Among those students is Baek Seung Yoo (Lee Do Hyun), an extremely introverted and seemingly troubled student who consistently ranks at the bottom of his class. Never speaking to anyone, Seung Yoo spends much of his time alone, choosing to take pictures with his camera, rather than interacting with anyone around him. Seeing Seung Yoo’s potential in math, Yoon Su is determined to help the struggling youth. With time and attention, Seung Yoo’s grades begin to improve but his success in math brings with it an entirely unexpected set of problems. With rumors of a teacher-student scandal now floating around school, it doesn’t take long for Yoon Su’s world to come crashing down around her. Reunited four years later, Yoon Su and Seung Yoo are determined to expose the corruption at Ahseong High School and regain Yoon Sun’s reputation as an excellent teacher. Will their efforts be enough to overcome such deep-seated corruption, or will their shared history only serve to destroy them further?