"The Deadly Years"
While on the planet Gamma Hydra IV, Kirk, Spock, McCoy, and Scotty are infected with an unknown disease that causes premature aging. The only member of the party unaffected is Chekov (Walter Koenig), who becomes McCoy's guinea pig while searching for a cure back on the Enterprise. A nifty idea with some poignant overtones, the story by David P. Harmon startles a viewer with the sight of these familiar folks rapidly graying, wrinkling, weakening, and suffering memory loss. At the same time, Harmon is careful to age each character as a unique individual. Kirk slows down more than the longer-lived Spock, while McCoy remains mentally keen, if physically brittle. As for poor Scotty, well...

The dramatic subtext in "The Deadly Years" concerns the perennial conflict over when and how to decide that someone has become too old to execute crucial responsibilities. In that sense, this episode feels constantly relevant and uniquely entertaining: let's just say some of these actors play "old" a little better than others. (Director Joseph Pevney has reported that there was a lot of conflict over who was stealing old-guy moves from whom.) With all this going on, one might not notice that guest star Charles Drake is a truly familiar face, having been cast in The Maltese Falcon and Now, Voyager. --Tom Keogh