"Return to Tomorrow" features perhaps the most definitive dialogue of Star Trek from Kirk regarding the mission of the Enterprise: "Risk is our business. That's what this starship is all about. That's why we're aboard her." Beyond that is an interesting story that never quite inspires, entertaining more than informative, and suffering from sorely dated silly special effects (those awful flames) and dreadful examples of "high tech" circuitry. Fortunately the "stringy" score provides memorable tones necessary to carry this love story of another sort.
Diana Muldaur makes her first in TOS appearing as Dr. Ann Mulhall, one of Starfleet's (and certainly Enterprise's) highest ranked women selected by Sargon as a "living receptacle" for his wife. (Why she wears a red uniform working as an astrobiologist remains an unsolved riddle.) Her performance is strong as she conveys both Thalassa's delight at returning to sensation yet also revulsion at leaving it for the proposed android body. She's good at that "robot lips" test too!
The rest of the crew provides support, not only McCoy and Scotty (both manage to work in an incredulous, "Are you joking?" Only to be reassured otherwise) but particularly Chapel who gets extensive screen time woven believably into the plot. Shatner as Sargon is impressive but besides the subterranean "introduction" scene we learn little about him except his magnanimity. Nimoy as Henoch however brings out a performance entirely from "the other side." Conniving, forceful, selfish, he is quick to exploit the skills of his Vulcan host (one wonders what sides of conflict could possibly be important in their armageddon).
The receptacles are simply styled yet calmly evoke tremendously superior technology. These katra-orbs are capable of near-instantaneous uploading, and they obviously function equally well even for relatively "dimmer" mental beings. What a pity we don't get a better subjective description concerning a stay in the receptacle beyond "floating in time and space." After their cataclysm Sargon's race for some reason opted for the orbs instead of corporeal existence, instead of using arbitrarily advanced android bodies to pass the millennia. (Androids of Mudd or Korby caliber would do nicely.)
Interactions with super beings happen all the time in Star Trek. Sargon and company certainly qualify but relatively they're not all that exceptional. (Consider the Organians for example that require no physical receptacle at all.) In a way a birth is witnessed as the survivors of Arret transcend their ancient longing for the material plane (and at half a million years, that's pretty "long") for a timeless place in oblivion together.