Park Chanyeol has been receiving letters. Letters that somehow encourages and make him want to work harder, better. He grows attached to this person who wrote those letters to him, but the thing is; he doesn't know who's this person is. And he is determined to find out this person.   

 There's something about you that triggered them, meaning; Exo-K, to protect you from any kinds of harm. Including harm from their own kind- their enemies to be exact, which is