Lee Haera comes to terms with the fact that she won't live past eighteen due to some sort of heart disease. Because of that heart disease, she's spent most of her sixteen years of life living in the hospital. 

Right on the eve of her seventeenth birthday, a boy roughly her age gets admitted into the same room as her. Now, the bed beside her won't be empty anymore. Not since Oh Sehun took it.

They began to share a mutual friendship over their problems. Hers being her heart, and his being his lung cancer. 

Both knowing that they're going to die without transplants, they create a bucketlist of stuff to do before they die.

Unfortunately, the transplant waiting list is extremely long and Haera and Sehun are at the bottom of the list.