After Matt is shot in the right arm (his good shooting arm), Matt is convinced that it's not good to be Marshal anymore, so he quits his job and leaves town for a more peaceful life. Along the way, he runs across a young fugitive who shoots well but refuses to kill. Also outlaws that have been after Matt for a long time go on the trail looking for him now that he's injured.
The Disciple has to be one of the strongest episodes of Gunsmoke that I've seen in my life! It does what a great TV episode should do, give the characters strong development, put them in extreme peril, and be really awesome about it. I really loved the relationship between Matt and the young man; the drama is excellent and it's emotionally powerful.