Independent and aggressive, Bok-soon is known around the neighborhood as a "psycho bitch" because of her weird behavior. She is "slow" and not very smart due to a developmental disability, but is nonetheless a brave young woman with a happy inner world. Bok-soon manages to make a living by running a street stall at the local market with her younger sister Eun-jeong, whom she loves more than life itself. The name-calling doesn't affect her at all, and she only cares about working hard and saving up money for Eun-jeong's education. Their relatively peaceful life is disrupted when Tae-soo, who lives alone in a deserted forest, kills Bok-soon's beloved sister because Eun-jeong may have stumbled upon the truth of his murderous lifestyle. Bok-soon is completely consumed by her grief, madness and uncontrollable rage. Despite her limited mental ability, she begins planning her revenge, and joins up with Na-ri, a little girl who is being chased by Tae-soo, after he killed her elder sister Yeon-Hee.