Joo Sihyeon, a successful 33-year-old CEO, has decided to return to high school to get his diploma. As Sihyeon tries to find a way to balance school and work, Eun Haim is worried about her new job as the older student's homeroom teacher. Haim soon realizes that's the least of her worries, as it turns out Sihyeon is the same man she had a one-night stand with in Canada just months ago! The two thought they would never see each other again, but who knew they would meet in a classroom like this?