"Third of a Man," as in general do all movies, offers a peek back at how mental illnesses were misperceived and at times punished through misperceptions. The story offers a redemptive conclusion.

Emmet Spile is a small town carpenter haphazardly raising his illegitimate son, Leroy. The child's mother, Helen Detweiler, has repeatedly refused to marry Emmet because of his violent nature. Unknown to everyone in the town, Emmet has a brother, Doon, whom he has committed to a mental institution. Doon is unable to speak and is terrified by water. When a guard turns a water hose on him, he escapes from the institution. He encounters Leroy in the woods and gains confidence through the youngster's gestures of friendship. When Leroy almost drowns while swimming in a lake, Doon overcomes his fear of water and rescues the boy. But his brother has organized a posse to search for him, and, discovered carrying Leroy in his arms, he is beaten by the mob, which thinks he means to harm the boy. Helen and Dr. Maxwell, the director of the asylum, intervene to save him, and he is returned to custody. Angered by his father's treatment of Doon, Leroy runs away to find his friend. When Emmet follows him to the asylum, Leroy locks his father in a room filled with mental patients. Although terrified at first, Emmet gradually realizes that no one intends to harm him. The incident changes his mind toward the asylum and he offers to visit his brother. As he leaves, promising to return the next day, Doon says "tomorrow," the first word he has uttered in years.—TCM.com