Beaming down to investigate the fate of a research colony that has failed to check in, the Enterprise crew discovers these researchers to be in a state of constant happiness, an effect caused by plant spores indigenous to the planet. Needless to say, Enterprise crew members are infected when the plants are brought aboard the ship, and before long Kirk finds he is the only person left who hasn't abandoned the Enterprise for "paradise."
Of course, the big story point of "This Side of Paradise" is that the spores allow Spock to experience full-fledged emotions and even briefly fall in love. Unfortunately, there isn't enough of an edge to the material. It's pretty bland. Nevertheless, it's probably worth the price of admission to see Spock hanging from a tree, and telling Kirk, "No, I don't think so," when ordered to beam up to the ship. And I must also admit the hilarity of watching Kirk push Spock over the edge into anger once he learns that negative emotions purge the spores. ("Your father is a computer!" has to be among the silliest yet more memorable lines in the TOS canon.)
Still, the best realization in this episode is when the effect of the spores is terminated, causing the research team leader to reflect on how the years have been wasted in a "paradise" that strove for no goals. Bottom line: entertaining, but pretty thin.