Orphan boy Yong-in has been stuck in the orphanage until he’s 15, while all his friends have already been adopted at earlier ages. He can’t wait to be adopted, especially to a nice house. Soon after, his dream comes true—he’s finally adopted. He believes the God must have answered his prayers. But it is quite early to call him lucky because the new family has a thoroughly planned reason for adopting him. They wanted him to appease their psychopath daughter Dong-ju’s violence! Will Yong-in be able to do that?

Dong-ju wants to become a sentimental poet to hide that she’s a psychopath. But she has a history of killng her newborn brother while he was asleep when she was 8 and murdered Dr. Kim, the child psychiatrist who diagnosed her as psychopathic. These are the facts only known by the family. “Yong-in, that’s why you should keep a close eye on your sister!” is what Yong-in hears all the time But it’s so hard to understand what a psychopathic sister thinks. He starts to learn about Dong-ju’s strenghts as they spend time together. ’What in the world are psychopaths good for?’

One day, a criminal who is wanted for murder snakes into the hometown of Dong-ju and Yong-in. Dong-ju asks whether killing the criminal sounds just, which confuses Yongin. But he has no other choice but to find the criminal for his sister because she said, “You’d better find him, or I will kill you instead.” Around this time, another uninvited guest appears before Yong-in, which is the police detective who pretends to be nice at first. As the time goes by, the detective reveals the true purpose of his visit. He tells that he has a map drawn by dead Dr. Kim, a psychiatrist, that shows the whereabouts of psychopath killers. The detective blackmails Yong-in, saying if he doesn’t make his psychopath sister to kill the criminal, he will arrest her for killing Dr. Kim and expose her true identity to the world. Why does the police want the criminal to be murdered instead of a proper legal punishment?