Lyric discussion by speccy94 

Cover art for Undercover Martyn lyrics by Two Door Cinema Club

It probably does have to do with the sister's boyfriend, I think it's about some guys who have a friend (say, Martyn) who likes a girl whose, "words... will melt in your hand," and, "spoke words of wisdom." These guys try to get Martyn to talk to her/make a move, which is his mission. Unfortunately, he always wimps out, opting to hide in the basement, making, "many surprises await you," sarcastic.

My two cents.

Song Meaning