The murder of a young school girl sets a small town in Korea on edge, only satisfied by immediately finding the apparent killer. Yoon Do-joon is the readily available scapegoat for police who have not had a murder case to solve for years. A mentally-disabled grown man, Yoon Do-joon signs a written confession, all the while proclaiming his innocence, during interrogation from three officers. The story of Mother follows Do-joon's mother as she does all that she can to prove her son's innocence and find the real killer.
Met with resistance on all sides from an uncompromising and lazy lawyer, a greedy friend of her son Jin-tae, to a discouraging detective, Hye-ja will stop at nothing to attain justice for her son.