"Mother" is a story about an overprotective mother (Hye Ja Kim) to her son Do Joon (Won Bin). When his son is accused of killing a school young girl, the mother is the only one who believes that her son is innocent. The mother hired a lawyer, but because the son is mentally retarded, unable to remember events that in fact the lawyer was so hands-off. The mother with a vengeance, trying to prove that the son is really not guilty. She paid a lowly police officer to investigate to investigate itself.
In the prison, the son trying to remember. That night when the murder is happen, the son was drunk. He later told his mother that night he saw an old man inside the empty house. The mother also recognize the old man, then she went to see him at his house to get more information. The old man says that he recall there was a young man throwing a big rock to at the head of young girl to death, when he would call a police by his home phone the mother deftly beats the man's head with an iron to death. The mother did it to cover his son's crime.