Love Songs Love Series X Years After: Our Promise.. My Friend

10 years after high school, five former schoolmates receive a letter from X, the only friend in the group who still lives in their hometown. In the letter, X mentioned a promise. Wondering what it is, they decide to return to their hometown, only to find out that X has disappeared. So they begin a quest to find him. During this time, they remember old stories, friendships, dreams, and love, as well as complexes. What is the mysterious promise of this group? And will they come across X?
(Source:
Korean = Wikipedia || Translation = MDL)