I've always had a problem figuring out the point of this episode. Is it the age-old adage about not judging a book by its cover? That surface appearance usually hides something else within? We are introduced to a new kind of alien in this one, a Medusan (refer to the myth of Medusa), a member of a race evolved into a formless type of energy that one carries around in a box. Colloquially stated, it's too ugly for humans to gaze at - it would drive a human being insane immediately. But, it probably has more to do with a visual spectrum that human eyes can't handle, rather than the traditional concepts of beauty and ugliness. Well, maybe that's it right there: yes, maybe there is no real beauty or ugliness, except the prejudices we humans project onto everything. There's a brief discussion of such during a dinner scene in this episode. Not too profound, but we're still exploring facets of human nature most other TV shows never touch upon.
Like with many 3rd season episodes, this one is afflicted by a slow pace in spots, i.e. it's deadly dull in places. Case in point: one scene has Kirk trying to distract Miranda, the alien's human associate, while Spock proceeds to the alien's cabin; we see Spock walking down the ship's corridor; then back to Kirk & Miranda; then back to Spock, still in that corridor. Suspenseful? No, just slow. All the male crew members seem really taken with this Miranda character, even though she's a real cold fish - ah, yes, she hides an ugliness within her, despite her surface beauty. Well, I never really felt there was this great ugliness in her, just an uninteresting self-absorbed personality. The action stuff is well done: the p.o.v. of a crazed person is shot with a disorienting camera lens, lending a suitably warped flavor to the scene. But when Spock's body is once again appropriated by an alien, you get the uneasy sense that Trek in its 3rd season is running out of ideas: it was done better in "Return to Tomorrow." The ending is somewhat anti-climactic. Take note that Spock wears the IDIC symbol on his uniform in this episode.