This is not a good premise, and it doesn't have a satisfactory ending, but so many other things are right that it is a simply marvelous episode.
The meat of this episode, like so much good Trek, is a deep exploration of our three main characters and their complex relationships. The characters are in a situation where we find out exactly how they each feel about their role and loyalty to the others. McCoy wins, but only because of a technological advantage. Clearly, each of these men is willing to do anything -- even die -- for the others. While this has been implied and assumed over and over throughout the series, here it is tested, and the results are memorable.
Each actor is given a chance to shine, and does. But the linchpin may be a stunning performance by Hays as the enigmatic Gem. With not a single word of dialogue, she has the task of creating a character that the others feel a need to protect, and that the audience can also feel for. Much credit must be given to the casting director, since Hays is utterly perfect for the role. But she herself does, along with her director, give a performance of great nuance and depth. She may be one of the most successful guest stars the series had.
The author of this episode is a complete mystery, not even listed in the IMDB. This suggests that it is a pseudonym, although for whom it is impossible to know. It has the feel of a money-saver since much of it takes place on a bare soundstage. The planet shots could easily have been done on a planet set created for a recent (or forthcoming) episode, allowing them to be shot earlier (or later), and then the stage simply stripped. The savings allow for many more special effects (Vian transporter and force field).
While going without a set (or at least without set walls) is risky in the wrong hands, here it is done beautifully. As with "Spectre of the Gun", the partial sets convey a strong sense of the temporary, which works for the situation -- a temporary lab set up on a dying planet. Unfortunately, the situation itself is poorly-considered. As I understand it, the Vians can save only one planet, and they're trying to figure out which one. To do so, they plucked one member from each planet and subjected him/her to a series of horrible tests trying to decide which species has the best ability to develop if transplanted. Scientifically sound it ain't. It's actually quite arbitrary and cruel, making it a little harsh as Trek premises go. And the ending is utterly unsatisfying because it offers no definitive answer to the question, "Has Gem done what she needed to do?"
I have read that an early draft of the script had McCoy dying, and Gem giving her own life to save his. The censors understandably objected to the blatantly messianic implications and demanded modifications. This is just the type of modification which could cause a writer to remove his/her name from the credits. In fact, without this seminal moment, the episode ultimately fails to deliver on what it is trying to do or say. With the basic premise requiring Gem to sacrifice herself wholly for someone else, we are left unfulfilled when she does not. By all rights, she has indeed failed the test and her people. This would be a more satisfactory (tragic) ending than the one we are given, which has no comment on whether she passed or failed, only that the testing is now complete. The smiling Vians may be intending to signal that she passed (or is being cut a break because of something Kirk said), but the ambiguity is lazy. It mars the episode and feels cheap (again, this is the style of much of the third season).
It's a pity, because almost everything else clicks perfectly. With the exception of the Vians' regrettable silver robes and big re-run heads (possibly recycled from the first pilot!), the production values are outstanding. Most notable is another Duning score which must be considered a classic. He makes evocative use of violins and harp, then tasteful use of the still-novel synthesizer. The lighting effects are outstanding, as is the masterful staging by Erman, a first-time Trek director (who later directed memorable installments of the mini-series "Roots"). (We may reasonably wonder why Kirk's shirt was off and McCoy's was not. We may also wonder why McCoy is clearly wearing a pinky ring in one shot.) The simple but intimate staging (looking much like a theater-in-the-round production) leads to a stunning shot where Kirk actually carries McCoy's limp body and lays it on the bed gently. It's an incredibly intimate moment for these characters, and handled with tremendous restraint by all.
It would not be unreasonable to rate this among the top Trek episodes for production, acting and script execution alone. But the story, a hallmark of all great Trek, is lacking. And the creative team did not have the fortitude to make a decision about the ending and stick with it. Sadly, those choices, while not fatal, diminish the whole thing.
Rating: Middle Upper (3)