This series follows three couples and focuses on two primary characters. Orphaned at a very young age, Gui, a fourth-year engineering student, has always had to work hard to support himself. Despite the many difficulties, he has kept his soft demeanor and gentle smile as well as his empathy for others. Whenever someone struggles, he is always one of the first volunteers to help. Solo, a freshman music student, has everything: money, fame, good looks. But he lacks a smile. After his mother’s early death, he lost his happiness. Nothing could fill this missing hole in him until one late rainy night he stumbles into a café for coffee. At that moment, their two worlds collide. For a cup of hot milk and a warm smile, Solo returns to the cafe every night. And Gui finds himself waiting for his late-night visitor. Night after night, the young men continue meeting at the same spot and at the same time. As they see each other more frequently and their lives overlap, the two become one another's breath…a necessity, just like Oxygen. (Source: Soju at MyDramaList) ~~ Adapted from the novel "Oxygen" written by Chesshire and illustrated by Suki.