Some of the best episodes of "The Twilight Zone" are stories like "The Lonely" - few characters, no complicated set-up/resolution, and an examination of a universal theme. In this case, the theme is loneliness, and the human need for companionship.
Jack Warden knocks this out of the park as Corry, the prisoner exiled to a barren world. John Dehner, one of the great unsung character actors of the golden age of TV, is his benefactor who provides Corry with an unusual way of passing the time. There's no "gotcha" twist ending to be found here, but the finale is still powerful and chilling. Warden's reaction to the sight of his companion's true "face" is amazing - simulatenously expressing dismay at the loss even as he knows he's about to get what he has wanted for so long.