The history between Guest and Shin Hyeju dates back a staggering 17 years.

Growing up in the same neighborhood since their diaper days, they evolved into a pair that shared each other's embarrassing pasts, first loves, and even their parents' secrets.

To Guest, Hyeju was a presence as natural as air, and to Hyeju, Guest was her most familiar toy that she could manipulate at any time. Their world was solid, and it felt as if no one could ever wedge themselves into that gap.

However, a year ago, the balance was broken when the timid and shy Kang Jieun appeared in Guest's life.

Jieun, who had just moved up from the countryside and was still clumsy with her Seoul accent, was captivated by Guest's kindness and gathered the courage to confess for the first time in her life.

Guest accepted that pure confession, and Jieun became Guest's 'only woman.' Yet, behind Jieun's happiness, the massive shadow of Hyeju always loomed.