An Innocent Sin

Thinking she's all by herself on the last train home, Nayeon feels the urge to enjoy herself by fantasizing about her desires. However, a reflection from the train windows show that a mystery man is on the same train. He comes up to her and offers his business card. At the thought of seeing this handsome stranger once more, she takes his offer and arrives at a place called Peach Puff. What is this place? What are these people doing?