Waiting for another spring day to come, so I can meet you again

Day by day, a former teacher is living her life with an empty space in her heart. Every night she dreams about someone who is waiting for her, but she can't reach that person. She wakes up crying, holding herself from falling apart.
She just knows that one year ago, on a beautiful spring day she visited a country from Europe and since then she struggles to bear each day.
On the other side of the story, there is Kim Seokjin, the visual of Bangtan Boys who is dealing with the loss of his beloved one, and trying to find a way to get her back.
Who is his loved one?
Who is the boy from Melody's dream?