Criminal psychologist Seonkyeong receives an unexpected call from Yi Byeongdo, a serial killer whose gruesome murders shook the world. He wants to be interviewed for the first time and he will only speak to her. That same day Hayeong, her husband’s nine-year-old daughter from a previous marriage, shows up at their door after her grandparents, with whom she lived after her mother passed away, died in a sudden fire. Seonkyeong welcomes her into their home but becomes gradually unnerved as the little girl begins acting strangely. At work and at home, Seonkyeong starts to unravel the pasts of the two new arrivals in her life and discovers startling similarities.