Seowon has weak lungs since birth, they had to leave the city and live in the forest with her father. One summer, Seowon's quiet life is disturbed by her father's friend and his son, Aiden. Seowon easily opens her heart to Aiden who loves her sincerely, but when Aiden approaches her as more than just a friend, she feels embarrassed but can't refuse him. Is it because Aiden keeps Seowon's secret to himself? In the end, 'they' cross a 'line' that shouldn't be crossed. "You mean, did you run out of breath when we kissed?" After spending the summer together, an unexpected 'problem' arises, and that 'problem' is what brings them back together !!