Taeyeon, D.O and Baekhyun were the "Perfect Three" in their school (aka Shinhwa University). They were literally "brothers and sisters". Their gang name was called the Perfect Three. As a result, everyone in the school respected them, and treated them like "queens and kings". But they weren't mean, though.  All was going well until one day, Baekhyun became a playboy. He wasn't the old Baekhyun. At first Taeyeon and D.O didn't think it was that serious, and Baekhyun was thinking

You left me. All alone.  Mark my words Do Kyungsoo. I will always love you until the day I die.