Remember that MASH episode where Hawkeye has a jeep concussion and tries to stay awake by talking, even though in a hut with villagers who don't understand English? The entire show consists of one camera, a few background extras, and Alan Alda, spewing non-stop dialogue. Total cost of the episode, $1.98.
That's what you do when you've blown your budget. You film an episode consisting of just a few actors, using sets and props that already exist. No money spent on guest stars, new sets, or special effects.
Voyage must have really blown their budget by this point in the season, because "Sealed Orders" is just such a cost-saving "bottle show". They even cut the budget for smoke bombs and sparklers! I counted only one smoke bomb (in the missile tube) and two sparklers (the computer and the Circuitry Room).
The actors spent the entire episode running through the corridors. Missile Room, Crew Quarters, Control Room, Circuitry Room. Oh, and a brief scene in the Flying Sub. Crew members kept disappearing at intervals (which saved on actors' salaries).
In the end, no real explanation, other than a weak theory about hallucination-inducing gas.
Yet another example of why I find VTTBOTS so difficult to watch. It boils down to Irwin Allen's contempt for his audience. He seems to think that we're all too stupid to understand that he's just wasting our time, filling the hour with nothing more than "running and jumping" actors, and no real story.
We spent an hour watching this show, waiting for it to make sense, waiting for an explanation, waiting for the story to come together. And then suddenly, the hour is over, the show is over, and we realize that we've been suckered into watching a nothing show, about nothing, in which nothing really happened.
Except that we saw the commercials from the show's sponsors. So, at least the ABC network made some money from the airing of these commercials. I guess that was the point all along, eh?
EDIT: Someone suggested that Voyage's budget may have been cut due to excessive costs on other series that Irwin Allen was producing at the time. If so, this is a mitigating factor, but also calls into question the advisability of robbing Peter to pay Paul.