the end of episode 4 gives us the first puzzle pieces that fit together. it seems implied that doctor has a history…
Exactly, the doctor and Jeong-woo’s friends (nurse, detective and the other guy) are somehow involved in the murder and disappearance of those two girls.
I think the killers are most likely the ml’s friends because they get defensive whenever he asks them questions. Also, when the forensic doctor mentioned the missing item underwater, Yang Byeong Mo's expression looked suspicious.