Yeonjun just recently received his teaching degree and today is his first day of work at a high school in Seoul. Yeonjun isn't nervous, he's dressed casually and is confident, because this isn't the first time they've had to teach math.

Everything started well, he was greeted by his colleagues, the students looked at Yeonjun with admiration because he was still young and understood them. Only when he entered the next class, Yeonjun saw a very familiar face...

It was the boy he had sex with at the bar yesterday. But how could this boy be at school? He told Yeonjun he was 20. Or was that a lie?... Yeonjun knows one thing for sure: he's in big trouble.