Lee Myeongyi has been burdened by pneumothorax his entire life and is tired of the unwanted attention it brings. The unwanted hyperventilating and coughing fits have caused him to be excused from physical education and military service, making him feel like a social outcast due to his curious classmates. However, Han Seonho, the class president, shows compassion and understanding towards him, leading to a growing affection from Myeongyi. Years later, Myeongyi attends his high school reunion with the sole desire to see Seonho again. His hopes of a mutual attraction are shattered when he spots a wedding ring on Seonho's finger, and his classmates' inquiries verify the worst. Heartbroken, Myeongyi retreats to the smoking area, where he is unexpectedly joined by Seonho. This leads to the inception of their forbidden love affair, which Myeongyi risks despite his delicate lungs. Thus, their story begins.