“Baekhyun-ah, it hadn’t been easy but our dream finally came true. It’s a sad thing that I am still alive. But you know, I don’t regret it at all because I met you and fell in love with only you. If I could meet you once more, or even if I knew a sad risk is waiting for me, I would still fall for you again.”

Byun Baekhyun, a boy who is told he will die before he’s 20, and Kwon Minhee, a girl who is in love with him.

The story begins when they are little kids. Minhee, whose father is a doctor, meets Baekhyun, who is hospitalized in her father’s workplace. They develop feelings for each other, but Minhee learns of Baekhyun's grave illness and that he will die before he turns 20. The distraught children make one promise for an uncertain future.

Time passes, and the girl becomes a young woman, and the boy, a young man. Minhee continues to support and love Baekhyun. On the other hand, Baekhyun, who knows his days are numbered, pushes away his feelings for Minhee and distances himself from her. He cannot stand to see her cry or hurt her more than he already has. Then Minhee meets another man who has feelings for her, and Baekhyun meets a woman who has the same illness, and their feelings for each other begin to waver.

Misunderstandings, unsaid thoughts, the frustrations and chagrin of youth… What will become of their love while the clock keeps ticking away in Baekhyun’s life?