The main reason this episode is probably rated fairly low is because you're watching writer Meyer Dolinsky drag the audience through a series of degradation games. the Platosians are another in a series of super-powered alien races, albeit low-grade compared to the Metrons, Organians, Trelane, Thalasians, etc. Basically Dolinsky hits the audience over the head with the idea that "absolute power corrupts absolutely"... except, of course, if you're the angelic Enterprise crew who all resist the urge for some sweet, sweet revenge at the end.
So after some minimal set-up with the usual coincidence required (the Enterprise just happens to be going by when Parmen happens to suffer a very rare life-threatening infection), it's watch the Platonians torture Spock and Kirk again... and again... and again. The torture is... well, torture to watch. As with most third season episodes, it's the Kirk & Spock Show, with McCoy along with the ride. Nichelle Nichols gets about as much screen time and a chance to do anything in the third season here as she ever will this late in the game. Yes, we get the first interracial kiss, with the sop tossed in that it's hinted at as a form of torture.
Liam Sullivan and Barbara Babcock do what they can with their one-dimensional stick figures in this morality play, but Michael Dunn, always a class act, shines here. He manages to rise above the mediocre scripting and give some real human reaction to the situation. His character doesn't really add anything to the situation, but whether it's Alexander reacting to the torture, or swearing vengeance, or his last look of joy and happiness at the end, Michael Dunn is always at the top of his game.
Overall a simplistic "power corrupts" moral message but so cloaked in allegory (by making the bad guys telekinetic meanies) that if there was any real message or real-world allegory to be found, it's deeply buried. Meyer Dolinsky has done better, but he should have stuck to spy, cop, and detective dramas.