Criminal psychologist Seonkyeong receives an unexpected call one day. Yi Byeongdo, a serial killer whose gruesome murders shook the world, wants to be interviewed. Refusing to speak to anyone until now, he asks specifically for her. Seonkyeong agrees out of curiosity. That same day, Hayeong, her husband’s 11-year-old daughter from a previous marriage, shows up at their door after her grandparents, with whom she lived after her mother passed away, die in a sudden fire. Seonkyeong wants her to feel at home, but is gradually unnerved as the young girl says very little and acts strangely. At work and at home, Seonkyeong starts to unravel the pasts of the two new arrivals in her life and begins to see startling similarities.