Kirk, Spock, McCoy, and Scotty undergo accelerated aging after being exposed to radiation on a planet, but Chekov isn't affected and Dr. McCoy must find out why in order to cure the others.
Air Date: 12/08/67
Director: Joseph Pevney
Artificial aging is a plot device in a few Trek episodes -most notably "Unnatural Selection" with Dr. Pulaski in TNG and "Distant Voices" on DS9. This one stands as, I think, the best of the three. The danger to the crew seems real and the acting as the main characters age is great - specifically McCoy..."Don't stand there jawing, Spock..."