During a ride on the subway, Jeong Won sees two young girls asleep on the train. He notices that they are alone, but he has to get off on his stop. The next day, he learns that the girls were poisoned on the train. When he arrives home, he keeps seeing the girls. They keep haunting him and it's slowly driving him crazy. With the help of Yeon, the only other personable to see the children, Jeong Won tries to solve the mystery behind the two ghost children. Along the way, he finds out things from his past that he has forgotten.