When Sulli and Minho were 18, before Minho left for the states to study, Minho has a made a promise Sulli that when they're the right age, he'll come back and marry her. Giving Sulli a ring, and telling her that to wait for him, Sulli hold onto that promise 

5 years laters, Sulli still kept that promise. Sulli turned down any admirers that confessed, for she thought that Minho will be true to his words and not disappoint her. After not having any connections for the past 3 years, Sulli don't know where's Minho or what he is up to

As fate starts to play with them, they once meet again. But to Sulli, it looked like the promise that Minho made to her, it seems like he forgot all about it. To Sulli's suprise, Minho doesn't look like he knows Sulli after all the years they have spent together. Throughout all of this, Sulli still has high hopes and is still waiting for him.

[Flashback]

"Bubba, wear this ring and wait for me. I'll be back soon  and when I come back, I'll marry you. I promise you. Don't forget me and our promise. It won't be long, the time I will spend in the states will be over before your eyes. Always know that I love you. I love you so much." Minho says, as slips a ring on Sulli's finger and kissing it 

"I'll wait for you. No matter how long it will take, I'll be here waiting. Come back soon, I'm going to really miss you. While I promise to wait for you to come back,you have to promise me that you're going be true to your words. Bubba, take care, huh? Don't starve yourself, don't stay up too late, do your studies well, as I will do the same. I'll be keeping your promise. I love you, now and forever." Sulli says, as she hugs Minho and buries her face in Minho's chest 

Giving Sulli one last kiss and hug, Minho left.