Mark's saddle cinch snaps while riding and he falls, leaving him paralyzed from the waist down. Lucas makes a dangerous trek to Mission Springs to treat Mark with mineral baths. Unknown to Lucas, there are three escaped convicts in the area.
The main problem I find with this episode is that the writers tried to cram too much material into the 25 minutes of episode. The idea of Mark being paralyzed is an excellent one, and would have held up the episode on its own. However, the addition of the run-of-the-mill escaped convicts brought down the quality of the story. We end up not feeling as much sympathy for Mark as we should because of the distracting sub-plot of the criminals. After all, Mark's paralysis is the reason they are in Mission Springs, but Mark is hardly seen, and says very little.
I was also disappointed by the performance of Chuck Connors. He came across as forced and stiff while interacting with Johnny Crawford. Maybe after all the years the two had spent doing father/son scenes, he felt he didn't have to try as hard? His cry of "Mark, stand up!" played as cheesy, not empowering.
Nevertheless, it is the Rifleman, and there are worse episodes. I recommend seeing it anyway.