Written by John T. Dugan under a pseudonym, this ship-based bottle show about God-like creatures interacting with the crew of the Enterprise the unique case where the conflict isn't between the crew and the aliens. The real drama is an internal conflicts the aliens bring with them, with the sci fi story exploring what it would be like to lose your corporeal body for so long, you forget what it's like to feel or to touch your lover, only to be overwhelmed by the sensations as you borrow a body to build a new android vessel for yourself.... which you begin to feel isn't enough.
And who needs guest stars to pull this off when you have talented regulars? James Doohan voices the only alien to speak outside a body, Sargon, and the "body borrower" plot allows Shatner and Nimoy to pull double duty, letting Shatner play Sargon in Kirk's body and Nimoy to play Henoch in Spock's. Unfortunately, Nichelle Nichols is not used for the third alien, Thalassa. (Later, in the animated series episode "Bem", Nichols knocks it out of the park voicing an entity with the same sort of voice as the aliens in "Tomorrow"). With Nichols being underutilized in the second season, it would have been a nice opportunity to give her two parts. The problem, however, is that the script requires Thalassa and Henoch to do a lot of embracing and kissing, and the show isn't ready for to dive into interracial PDAs (yet). Enter Diana Muldaur as Dr. Ann Mulhall, an Astrobiologist and perfect smooching partner for Kirk/Sargon. (Shatner probably cast the part himself). Muldaur, despite being a knockout beauty, seems to have more sense than most of the female guest stars, and gives Mulhall a professionalism to ground the character while giving Thelassa the internal struggle.
To help differentiate the possessed crewmembers from their original selves, the sound editors add reverb to the "alien" voices, which doesn't always make much sense from a logical standpoint (shouldn't Kirk's voice sound the same whether he's possessed or not?) but works well if you can accept the conceit. Meanwhile, George Dunning (who previously scored "Metamorphosis") gives the episode quite a bit of new music, with some themes never reused, and director Ralph Senensky uses strong, less graded colors than usual, making the whole experience rather unique for the series.
With a good story as his outline, Dugan provides a well written teleplay (which would go on to be nominated for a Writer's Guild award ), filled with sharp dialogue and interesting thoughts. (Unfortunately, he gives Kirk a messed up Apollo reference, having the Captain confuse the first Apollo mission with the first lunar mission. Neither had happened yet when this episode aired, if you accept that we're talking about manned missions and you don't include Apollo 1, which never launched; but even back when this episode was written, it was no secret that the first manned Apollo mission to launch would take place entirely by the Earth, with lunar missions being saved for later. Kirk would no better, and so should Dugan).
What really sells the whole thing, however, are the performances of Shatner and Nimoy, who clearly enjoy stepping outside their normal roles to play other characters. Shatner uses the opportunity to emphasize his physical movements (looking much like the Shatner comedians like to parody) while Nimoy uses it to create a more understated, grinning bad guy that steals the show.
There are better episodes, but this one is sort of a hidden gem.
Remastered version: This is as basic as can be, offering a new CGI shots of the Enterprise in orbit. (Originally, the episode borrowed these shots from "The Deadly Years").