(I am resuming my goal to review every episode of the Original Series. As if the third season wasn't enough of a slog in the first place, my DVD set had two damaged disks (one of them containing this episode) and I had to replace them. Oh well, back on track…)
"The Lights of Zetar" is another one of those average-at-best third season episodes that has nothing particularly outstanding about it.
Memory Alpha, a planetoid that is a library for Federation records, is a very interesting concept, but deserves a story more solely devoted to it – here it rather just ends up as the standard 'endangered location of the week'. The title of Memory Alpha is now used as the name of a popular 'Star Trek' wiki.
I don't find the love interest between Mr. Scott and Lt. Mira Romaine to be convincing at all. Although the late James Doohan appears to do his best with the material he is given, but there is little real spark between the pair. The interest seems purely one-way, with Scotty, I have to say, rather appearing as the lusty old man after the sexy young girl.
...Following on from which, there are some very out of character moments for Scotty. Yes he is in love with Mira, but I couldn't believe that he would deliberately disobey Kirk's orders and follow her to sickbay; nor would he not report Mira's strange condition that soon emerges.
As with Scotty's previous love interest in the second seasons "Who Mourns for Adonais?", this is the last we ever heard of the girl (although several non-canon novels have included Mira).
When Mira becomes possessed by the energy-based aliens, things started to become a tamer version of the first season's "Where No Man Has Gone Before". However, the story didn't follow up on the quality of that episode, and I didn't really buy the later moments of this instalment. I didn't fully grasp the climatic scene in the pressure chamber, and it didn't really feel like a pleasing nor believable solution to the premise.
All-in-all, not really an outstanding episode. In seasons one or two, this would have served as filler offering at best; unfortunately, the third season is rather full of these middling stories, and is a prime example of why the third season is widely regarded as the weakest of the three seasons of the Original Series.