Like "Arena", this episode is another story by Gene Coon that challenges the preconceived ideas of televised science fiction, this time eschewing comic book violence for thoughtful discovery.
Taking place almost completely beneath the surface of the planet of Janus IV (brought to life through studio sets), this Kirk/Spock story is built upon the sci fi idea of silicon-based life, embodied by what looks like a bad slice of pizza. For the 60s, it's an advanced idea and an advanced look, particularly for a series that has so many humanoid aliens, with the original series presenting many, such as in "Miri", "The Return of Archons" and "A Taste of Armageddon", as regular looking folk.
As in "Arena", the script embraces Kirk's entire character, faults and all, turning his eagerness to get the job done and the protective nature toward his crew into blinding aspects that cause him to misread the situation and choose the wrong course of action. As with the Gorn, he eventually ends up face to face with his enemy, only this time he realizes the monster is himself. Shatner plays it all perfectly, beaming down to the planet with a smug attitude that changes the moment one of his own men dies. (The look on his face as he processes the death is priceless. Let it not be said that Kirk doesn't care for a redshirt death! Well, not this week at least). And then when he meets the creature at last, he finally puts the puzzle pieces together and realizes what's going on, forcing him to reassess his outlook and his decisions. Giving Kirk this arc offers us a chance to see him learning and growing, and through this we can feel much closer to him than if he has to be perfect.
The secret sauce, however, is how Coon and Nimoy are able to use Spock to bring out the Horta's personality and turn her into a three dimensional character. Future Star Trek episodes and movies take note and borrow the idea for other creatures, but Nimoy hits it so note-perfect the first time, the concept is never improved upon.
It all shows what a long way Star Trek has come in its first season. After beginning with a simple monster hunt in "The Man Trap", the series approaches the finish line with a much improved version here. And to think that the season's greatest episode is still to come! It's quite a beginning for the one that started it all.
Remastered: Of all the matte paintings of the original series, none is in more need of an upgrade than the one that begins this episode. Poor Albert Whitlock either mailed this one in or he was too rushed for time to create something more realistic; either way, while it's a fine piece of art, it looks so fake that even back in the 80s when I was watching the episode, poorly taped on VHS, on my 12 inch analogue television set, I thought, "Oh, come on!" when I was asked to accept that what I was looking at was a real underground factory and not an abstract product of Vincent Van Gogh's imagination. Fortunately, the show seems to feel the same way, since the matte painting only pops up one other episode ("The Gamesters of Triskelion") and only then in the far background of a live action set. For the remastered version, CBS Digital completely replaces it with a new matte that uses the same concepts but brings them alive in a much more realistic way. In its first appearance in the episode, it even includes an orange suited worker in the lower left entering a tunnel, which then cuts seamlessly into the live action where he continues on; it's amazing to think the shots were created forty years apart. The remainder of the episode only uses the matte painting behind an office window, and for this use, the original matte painting works ok, because it's far enough back to hide its flaws. Nonetheless, CBS Digital even replaces this with their new matte painting, which is painstaking rotoscoping work, with a moving camera and moving actors (occasionally blocking the view) to contend with. Like the window at Starbase 11 in "The Menagerie", the work is so well done, you'd never guess there was any work done at all - which is the best kind of digital upgrade.
Other than that, there's not too much of note. We get an upgraded planet (closely matching the original, which was lifted from "The Man Trap") and an upgraded ship; and there's a shot of the "monster" coming through the wall that's been fixed so that she now emerges from a spherical tunnel matching the other tunnels she's made. (In the original, she emerges from a tunnel the shape of an upside down "u", going all the way down to the stage floor to allow the practical effect to emerge at ground level).
The original ending includes another one of those slow fades from the characters to the ship that causes trouble for this type of project (with all those frames unusable), so CBS digital has to crossfade from the bridge to their new shot of the ship as Kirk is still giving the order for warp speed. It would be better if they could delay the crossfade half a second, but they do the best they can, and it works ok.