Just as the regular school day ends, and the moon shyly rises, Kim Jongin aka "Kai ", the mysterious hot freshman that's always to talk of the school, gets a call from a unknown phone number. He stares at the screen, slowly pressing the answer button.

"Uh-- ... Hello?"

He is not answered by a "Hello" or "Hey", but is answered by an soft voice singing Angel by the sub-unit EXO-K. 

For some reason, Kai feels like he could listen to this voice for months-- No, years. He just closes his eyes, wishing the song lasted forever.

To his disappointment, the song ended. But the owner of the voice then introduced himself

"Um. Hi. Um..
I'm D.O, Bye."

D.O then ended the call right after, again to Kai's disappointment. Kai wanted to know more about "D.O".