A widow (Kim Hye-ja) resides with her mentally challenged son (Won Bin) in a small South Korean town, where she scrapes out a living selling medicinal herbs. Mother and son are plunged into a nightmare when the body of a murdered young girl is discovered. Circumstantial evidence indicates the son's involvement, and he becomes the prime suspect during the sloppy police investigation. Betrayed by the legal system, the mother takes the law into her own hands to clear her son's name.