Sooyoung is a girl who got to married with a guy that she never know.....their parents made the promise because Sooyoung's late father save Siwon's father from one accident when Sooyoung at 5... 

they had promised to each other since they always meet after that and they get knowing to each other's family...
from there's Siwon's father start liking Sooyoung..and keep a dream..that she will be his daughter-in-law one day....[since he's already have Siwon as his son]

that's why they made promised..and a few years later,Sooyoung's dad was died...Sooyoung's family has encountered many financial problems..that's make them to move away from there...and Siwon's family tried to reach them but failed..until..they got a news that Sooyoung and her mother still alive and live in Seoul... they move to Seoul and tried to reach Sooyoung's family back...and..finally..they did it...they met again..