|via Danger Mouse|
Memory Alpha summary
"The Apple" has the distinction of being the prototype for one of the series' favorite tropes: an alien culture that is controlled by forces it doesn't properly understand. As in: if Kirk exposes this thing, the culture is freed from tyranny. (It's a trope that recurs throughout the franchise but is best known as something that frequently occurs in the original series.)
And so it's one of those Violating the Prime Directive with Impunity episodes. But we feel all warm and fuzzy about it because our familiar characters have been put in mortal peril because of the local lunatics. The whole thing is actually Gene Roddenberry's critique of religion and/or Christianity (that's where the title comes from, which is a fairly presumptive way of saying that Kirk introduces true wisdom to the Garden of Eden; the devil we know, as it were). It's a fairly well-known episode of the series, although not in the good way that a great number of second season entries are.
What's more significant about "The Apple" is that it's another prototype entirely: it's the secret origin of the Red Shirt Curse. It may be worth noting that Kirk is actually pretty distraught over the alarming series of Red Shirt deaths ("Red Shirts" being the security officers, who wear red tunics, who routinely die on away missions). John Scalzi's Redshirts is a whole book that looks at the phenomenon, won a Hugo and everything.
In that sense you could probably add the episode to a kind of list of essentials. But only if you really want to.
franchise * series * essential * character