Donghae and Yoona are classmates since gradeschool. But they always ignore each other. Yoona's father broke up with his wife and marry another girl. Yoona investigates who was the girl, but she finds out that the girls is Donghae's mother. Yoona will make Donghae fall for her and when Donghae confessed, Yoona will break his heart. But what if Yoona will also fall for Donghae?