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?

Do Ahyeon is a total nutjob, at least according to her boss, Heeseong. Not only is she blackmailing him into sleeping with her for fun, but she's got a phone full of guys she's ready to get down and dirty with at a moment's notice. Sharing is caring in Ahyeon's world, and there's more than enough of her to go around. But while wading through her diverse cast of boy toys, from total players to absolute control freaks, Heeseong begins to realize that he has real feelings for Ahyeon. And now, he won't settle for being second... or third, or even fourth best. It's a love-hate relationship of the most complicated kind, but he's willing to do whatever it takes to win this wily woman's heart.