“I watched the sky all those times as it faded from day, to night, to day, to night and back to day again. From blue, to orange, to the darkest of the pitch blacks and when it went to repeat the cycle again, and again, and again. I watched the sky and envisioned the two of us being able to run under it together again, someday.”

Lee Taemin had unexpectedly left Zhang Xiaoying and their perfect relationship 6 years ago. 6 year later, now, however, he returns to Seoul, the city of their memories, to claim her as his again. They were made for each other, after all, so it's only right for him to come back and claim the girl who's rightfully his, right?

But what he doesn't expect, is that perhaps his destined one has moved on,  perhaps she already has a new boyfriend, and that perhaps she doesn't appear to love him like she once did.

So what is a boy like him supposed to do, if his partner at the other end of his red string seems to have forgotten that he was the one at the other end of hers? What is a boy supposed to do, when the girl he is wholly and completely in love with, seems to have forgotten about their red string of fate?