This is pure speculation on my part, but I think William Welch attempted a "serious" LIS script loosely based on the "id" monster from Forbidden Planet". I don't know how much of his original story was changed at the editing stage, but the result is a very off-beat, bizzare episode. Not in the usual campy sense, but in a Twilight Zone esque kind of way.
There ARE a few campy lines, and the scenes of the giant hand coming down over the viewport are laughable, however, the scenes on the fog shrouded "planet" are very effective. As are the tense climactic scenes on the Jupiter. The "monster" (a guy in a sheet) is also suprisingly well done, and conveys the primitive nature of the creature.
BTW, When the characters start disappearing I love the way they talk about splitting up and searching for them in the ship, as if it's the size of the Titanic. Nevertheless, a very atypical episode and one of the best of the series.