Star Trek's second take on Moby Dick is often called a unsung classic by fans, with the captain-driven episode offering some rare Kirk-backstory and featuring some of the most contentious Kirk, Spock, and McCoy scenes of the series.
There's no doubt that Art Wallace delivers a tight script, with all the elements of the story nicely set up and paid off, and many fans like it when Star Trek dispenses with morality plays and simply pits the good guys versus a monster. Ralph Senensky shoots the ship-based story with gorgeous cinematography, highlighting the colors of the Enterprise's interior, and he keeps the attention on a central through-line, lending clarity to the plot. Yet all the same, the episode doesn't crack top ten lists, probably because it lacks the urgency, plot development, and special effects of "The Doomsday Machine" and comes across as the poor man's version of that episode as a result. (It doesn't help that the script feels it necessary to justify Kirk's decisions before the end, which is the right thing to do for the series but makes for a less compelling study of obsession as a result).
The episode is probably most famous for killing off as many redshirts as "The Apple", although a couple of them (Leslie and Rizzo) return later in the series (with Rizzo appearing in the next episode).
So for Captain Kirk, it might a memorable adventure, but for the rest of us, it's somewhat forgettable, though somewhat unique and satisfactory at the same time.
Remastered: With the adventure spanning two planets and featuring a strange, gaseous creature, you might think there's a lot for CBS Digital to do here, but really it's just a vanilla episode needing a small facelift. The shots of the Enterprise in orbit, originally borrowed from "Where No Man Has Gone Before" and "Amok Time", are replaced with two barren looking planets, with the latter eventually showing damage caused by the climax of the episode. When the Enterprise flies through space at ungodly speeds to catch the creature, the upgraded shots show the stars whizzing by. Lastly, the special effects for the gaseous creature (and the ship's battle with it) in space are redone via CGI to match the practical effect of the creature on the planets.