Ga-yun had to choose a penpal for a social assignment.

It could be anyone she chose, random or not. 

So who does she choose? Why, Exo's Chanyeol ofcourse!

Even when she doesnt gets a response, she decided to keep writting.

Soon spilling all of her secrets, emotions, inner thoughts

It wasnt like anyone was actually going to read the letters, right? 

Chanyeol was a world away from her, why would he read the letters of a fangirl?

A fangirl with cancer ontop of that.