The Only Child is the first opus of a trilogy dedicated to Hayeong, a young icy psychopath. Both a victim and a criminal, she embodies the upheavals experienced by Korean society, which one wonders if it has not become a monster factory.
Yi Byeongdo is a serial killer. Guilty of a dozen homicides committed against women, detained for several months under high surveillance, he refuses any contact with the outside world. Until the day he asks to speak to Seonkyeong, a criminologist. Warned of the dangerousness of this manipulative criminal, she nevertheless agrees to meet him. A certain intimacy is established between them. At the same time, Seonkyeong’s husband asks her if they can take home the daughter he had from a first marriage. The eleven-year-old child has just lived through two tragedies, the suicide of her mother and the death of her maternal grandparents in a fire from which she miraculously escaped.
Soon, the little girl shows worrying signs of hidden violence. These signs only grow as the investigation into the death of her grandparents takes a new turn.