After watching the MV multiple times, my theory is: Soobin was overthinking if she's cheating on him that's why she followed her but the reality is, she's not. She was just planning a birthday party for him with his friends. The guy he met at the cafe is the same person beside her in the end. the other members was Soobin's imagination of what he will do if he actually caught her cheating. Taehyun at the train-ask who she's talking to, beomgyu at the cafe - touch her hand, yeonjun at the streets - make her fall in love again and Kai at the club - let her be happy. At the end, he realised that the other person was himself all along (there's no other guy) and it was all just anxiety.