Simple minded Karl is a good guy who is mentally slow. As a teenager he finds his mother apparently being raped by the neighborhood bully Jessie Dixon. Karl becomes enraged and picks up a sling blade that is on the porch and hits Jessie on the head twice killing him. Karl's mother is upset that Karl killed Jessie. Realizing that his mother was willing engaged in consensual sex with Jessie, Karl becomes further enraged and turns the sling blade on her killing her as well. He is sentenced to a mental institute. He is released years later. He befriends a young boy and his mother. Before long he decides that the boy's abusive step father needs to die. He ends up back in the institution.