Instability seems to be a requirement for scientific geniuses, except for Nelson. (And sometimes we're not too sure about him.)
Gantt's character reminds me of Captain Adams from the first season's "The Creature". Not only are both Naval officers, but their actions resulted in the deaths of numerous men. Both put all the blame on a "monster", and both were concerned with exonerating themselves. Gantt has somewhat more depth to his character, however--although he's still obnoxious.
Gantt's genius is set to making submarines (and other vessels, presumably) run faster, and he has developed an exceedingly dangerous, but effective, fuel, which he is testing in the teaser. The agreed-upon speed limit seems to be between 45 and 50 knots, but once there, Gantt brushes off his exec's concerns and continues cranking it up to 75 knots--25 over the limit. The hull seems to be handling the stress fairly well--but the instrumentation has not been designed to cope with such speeds, so they are essentially running blind. (The test sub lacks the Seaview's observation windows.) Nor can they send out radio signals, although they can receive them. Not that it would have mattered--Gantt chose to ignore the incoming message from a superior officer.
Sonar picks up an object ahead, and Gantt, without even looking at the instrumentation, assumes that it is simply a shock wave caused by their speed. Balter, the exec, who HAS taken a look at the instruments, orders the sub to swerve, but the order is instantly countermanded by Gantt. A few moments later, they crash full tilt into a giant web. The damage is extensive enough that they must abandon ship. Before leaving, Gantt takes a look outside and sees what looks like a giant spider floating in the water.
Spiders are a very sucessful life form, so it seems quite reasonable to suppose that some could adapt to underwater life--if perhaps not quite to that size. The special effects were well done, especially with distance shots and rear shots. Close-ups of the spider from a front view were a rather shaggy mess.
The scene jumps to the Flying Sub, returning to the Seaview after, presumably, a preliminary hearing about the loss of the test sub. We don't learn just how many men died, but apparently it was a fair number. Gantt is already placing all blame on the monster he saw. In spite of their own run-ins with various assorted monsters, Nelson, Crane and the rest of the Seaview's crew are rather skeptical. All of them seem to know Gantt's reputation.
Back on the Seaview, Gantt is sent to observe an experiment involving his new fuel. Gantt is very put out and impatient about this. Using extreme caution, Crane places a highly diluted sample of the fuel in a heavy cannister of sea water. Apparently an undiluted sample would have the power to blow up the entire ship. It seems to me that this fuel is too dangerous even without the added complications. However, it turns out that sea water under high pressure causes the fuel to explode and release a toxic gas. There are four cannisters of the fuel left in the test sub (hey, what about the stuff in the fuel tanks?) and the sub is at sufficient pressure to eventually cause an explosion that could conceiveably kill millions. To his credit, Gantt instantly acknowledges the danger, and they set about retrieving the fuel.
Nelson, Gantt, and Riley (Riley's role was increased because of Basehart's illness) take off in the Flying Sub, leaving the Seaview to catch up in a couple of hours. Having made it to the sub--and seen no monsters about--Gantt and Riley go down into the sub to retrieve the cannisters. Nelson remains behind because for some reason, the Flying Sub does not seem to be properly attached to the test sub's hull, and it tends to want to drift. (Well, they had to make some excuse for Nelson not being there.)
Riley hears a tapping sound, which Gantt at first dismisses. Hearing it for himself, he realizes that some one may still be on board. The hatch where the tapping comes from is jammed. Rather surprisingly, Gantt takes an incredible risk by using a few drops of his fuel as an explosive to open the hatch. The survivor turns out to be Bill Balter.
Coming back to the Flying Sub, they find that Nelson has apparently been taking a nap. The spider has returned, and wrapped the Flying Sub in webbing without Nelson even noticing. The spider starts to drag the Flying Sub away. Nelson prudently has Riley put padding around the fuel cannisters before they attempt to break away. The attempt fails. Nelson, knowing that the Seaview will be there shortly, does not try to use more power--the stress will be too much. Gantt, who seems to equate caution with cowardice, disregards Nelson (that's ADMIRAL Nelson, by the way--Gantt seems to have forgotten that) snatches the controls, and cranks up the power. The resulting explosions injure Nelson severely.
The Seaview approaches, and understandably pauses to assess the situation. Gantt again confuses caution with cowardice, assuming that Crane is afraid to send a rescue team. Crane, in fact, is already on his way.
The ensuing scenes are jumbled and rather claustrophobic, as the diving team begins cutting through the masses of webbing around the Flying Sub, while the spider, sensing something tasty in the vicinity, sends out a strand of webbing that appears to be prehensile, not to mention intelligent--it searches around until it finds one of the divers, then wraps him up. Crane has gotten on board, to find Nelson injured. Gantt says that "we" had an "accident", forgetting to mention that he was responsible. Riley looked disgusted, but did not try to actively contradict a superior. Having determined that the Flying Sub is still navigable, Crane orders Riley to take it back to the Seaview as soon as it's free. Gantt wants to stay and tackle the spider--indifferent to the two wounded men on board, not to mention four cannisters of explosive. Crane has to remind him just who is in charge (which I found very satisfying to watch.) Riley spots the captured diver, and Crane leaps for the escape hatch, nearly jumping out before he's had time to gear up again.
The second diver gets tangled up while attempting to free the first, and Crane requests an underwater torch to cope with the situation. Gantt brings it, no doubt pleased at the chance to confront the spider. The men are freed, the Flying Sub heads for home--and the spider heads for Crane and Gantt, scrabbling along the sea bottom and looking very scary indeed.
Seaview manages to distract the spider--not kill it--with a laser, although I would like to know how Chip Morton, down in the Missile Room, could see what the spider was doing. All four divers eventually make it back to the Seaview.
Crane is called down to the Sickbay to witness Balter's dying moments--and his confession. I found this irritating in the extreme. Balter had seen an obstacle and attempted to avoid hitting it, which was exactly what he should have done. He seemed to forget that Gantt immediately countermanded his order--and the crewmen should have obeyed Gantt. Even if Balter had panicked--and it sure didn't look as though he had--the fact remains that the crash would probably not have occurred if Gantt had run the test as ordered. Racing blind at 75 knots, they could have crashed into anything. It seems a pretty cheap trick to lay all the blame on a dying man. And they seemed to forget that Gantt was directly responsible for injuring Nelson. It would be nice to think that Riley and Nelson brought this out later.
Gantt is determined to kill the spider, and he wants to kill it himself. They could have easily dealt with the spider with a well-placed torpedo, but Gantt elects to be a hero by heading out by himself with a harpoon loaded with a test tube filled with his explosive. (The concern of sea water, pressure, and toxic gas seems to have slipped his mind.) The sea spider, who had been quietly minding its own business until Gantt and Company turned up in its vicinity, gets blown to bits.
Another rescue team fetches Gantt back in and he's taken to Sickbay. He suggests to Crane that they use his fuel to zip back to Santa Barbara at 90 knots--proving that he has learned absolutely nothing from the experience.