Min Haram has spent a decade trying to solve the problem of her unrequited love for her handsome neighbor, Yoon Sunho. But the math changes overnight when Sunho walks into her senior year classroom as her new homeroom teacher. His instructions are cold and immediate: they are to act like total strangers, and she is never to acknowledge their history. Navigating her final year becomes a minefield of jealous classmates and Sunho’s icy, professional stares that seem to erase ten years of closeness.

As the lines between student and adult begin to blur, Haram realizes that their Love Equation isn't as simple as she thought. Behind the closed doors of his officetel, the "extra lessons" Sunho provides tell a very different story from his classroom lectures. She spent years waiting to be his equal, but she wasn’t prepared for the intensity of a man who has finally decided to stop treating her like the little girl next door.