This canonical episode of the original series explores the universality (and non-universality) of love, and introduces the inventor of warp drive, Zephraim Cochrane. Although obsessive fans may find fault in some aspects of Cochrane's barely explored back-story, this is the episode that created Cochrane's legend and laid the groundwork for one of the franchise's best efforts - First Contact - among others.
Glen Corbett and Elinor Donahue are strong guest stars playing the bold personalities of, respectively, Cochrane and a terminally ill diplomat (Nancy Hedford) on a vital mission for the federation. En route, Kirk, Spock, McCoy and Hedford are essentially kidnapped by an energy being who they will come to know as The Companion. Once landed, they meet an impossibly young Zephraim Cochrane and hear the bizarre story of how he ended up about 200 years younger than he should be, alone with The Companion on a distant planet. Hedford will die if she doesn't get proper care very soon. Cochrane will die if he does not have some human companionship. The Companion may die without Cochrane. Our heroes are powerless to resolve these problems.
The episode is very story-driven and well written by the stalwart Star Trek legend Gene L Coon. It is probably Ralph Senensky's best directorial effort on behalf of TOS. Shatner, Corbett and Donahue all give nice performances, even if Cochrane and Hedford are not the most likable characters in the pantheon. The special effects for this episode are decidedly mediocre, but do not detract from the flow of story.
Recommended, even for non-fans.