On a Christmas eve, a lifeless body of a teenage boy is found in the water tank. The body belongs to an 18-year-old boy with mental disabilities, Joo Wol-woo. Despite clear signs of brutal violence, the social worker has the boy's grandmother, who is suffering from dementia, sign the death certificate stating the cause of death as an accidental drowning. Following the incident, the grandmother dies alone from distress, and after losing all his loved ones, the victim's twin brother, Joo Il-woo, is left on his own to seek justice. To his dismay, however, the world turns a blind eye to a teenage orphan struggling with poverty, and Il-woo finds himself completely alone in his journey to avenge Wol-woo's death: He determines that it is about time he has to take the law and justice to his own hands.