Hye-ja is a long ago widowed mother who lives alone with her only son Do-jun.
Although Do-jun is 28 years old, his mental capacity is far below someone his age and perhaps the reason for his many eccentric personality traits.
Late one night, Do-jun leaves a bar drunk and walks home. Along the way home, he follows a school girl down an alley, before a huge rock thrown his way chases him away. When he arrives home, Do-jun sleeps on the floor next to his mother like he always does.
The next day the police discover a murdered high school girl and a golf ball with Do-jun's name inscribed on it near the crime scene. With only this small bit of evidence, Do-jun is taken into custody and promptly arrested for her murder. His defense attorney looks for a quick plea bargain - which would require Do-jun to spend four years in detention - which isn't an option for Hye-ja and her only son. Faced with no other options, Hye-ja takes the initiative to prove her son's innocence.
