After witnessing a tragic car accident, Taeyeon is reminded of her past. This past had left such a deep and damaging scar on her that she feels obligated to fulfill the favor assigned to her. She, together with this stranger she just met, will make a promise that will completely change their lives. Will they be able to carry out the favor and keep their promise or will they not? Is this really love or is it just their guilty conscience taking over? Moreover, will something or someone else come in between them?