"This Side of Paradise" is the space hippies episode, just to make it perfectly clear from the start.
Actually, there is a fair bit more to know about it, such as this is the first time Spock (or any Vulcan in the franchise) loses his emotional control, and for that reason this is the main source of recommendation I have for the episode, for Spock himself although it could also be as a franchise signifier. Like a few of the earliest episodes, there's a flimsy tie-in with his past that becomes ignored later (unfortunately there are plenty of episodes from other series that share this trend, too) but serves as the hook. Basically, this is an episode that fans of the character would probably have greatly enjoyed, although there is better material elsewhere, in later episodes (such as "Amok Time," which is far more relevant to Spock's development).
The space hippies have a space spore that acts as their unintended drug of choice, which of course attacks the Enterprise crew as well. The way out of the predicament recalls the later Klingon episode "Day of the Dove," interestingly enough (which is a better one), although to be fair to the writers (generously so), the solution is basically in reverse. If that means much to you.
Although, all you really need to know is that this is an episode that has grown less and less significant as time has gone on. It's kind of a lousy version of the emerging hippy culture that clearly doesn't understand it much at all, which is unfortunate. But Spock gets to let loose a little! Anyway...
franchise * series * essential * character