What happens when you begin to grow distant from the very one person you planned on sharing your entire life with? 

Yoona and Luhan have grown up together---they've been with each other since the diaper ages, and now, they're finally in high school. 

Maybe it's puberty, as some say, but Yoona realizes that she's becoming weird around Luhan...

...she has to fix her hair, feels the need to wear shorter skirts, and can't seem to eat as much as she used to in front of him.

What...what exactly is happening?