The plot was one that would be reused a number of times. It's not too badly handled here.
First Venus probes, now Saturn. Seaview's crew probably ended up wishing the problem with the Saturn probe had been as simple as sabotage. Earth sabotage, that is. The returning probe is found to have some sort of growth on it, and they dare not risk contaminating the Earth with whatever it is. Bringing the probe in at a sharp angle increases the amount of friction, and therefore heat, and the growth seems to burn off. The probe lands in the ocean, and of course Seaview is the nearest vessel.
Diving down to retrieve it (some more excellent underwater shots) Crane and his men discover the growth has come back, and seems to be actively hostile. Despite the fact that the burning re-entry failed to destroy the thing, Nelson sends down torches for them to try burning it again. (It's no wonder it doesn't work the second time, either--Crane passes his lit torch right over a diver's arm, and the man never twitches.)
After hauling the probe aboard, they do some more decontamination procedures before retrieving the information stored inside. Nelson and Sharkey take this to the carrier Huron--first cautioning Crane to seal up the probe and declare a quarantine. Crane does so.
They toss in a bit of extra interest with the Flying Sub, demonstrating yet another new gadget--this one a special camera that can see through fog, allowing Nelson to safely pilot the sub onto the carrier. Presumably there is also another gadget which allows the wheel-less, skidless Flying Sub to make a landing on a hard surface (and take off again) but we don't get to see this.
It would have been a lot shorter episode if Riley, Kowalski, and the other two men had remembered that the room the probe was in had been quarantined. We see Riley walk casually past the probe, engaged in some routine work. The growth reappears. It was a lot better looking in space, and under water. Perhaps exposure to an oxygen-rich atmosphere causes it to bloat. It more or less engulfs Riley, who is too startled to cry out, and leaves him twitching on the floor. Recovering (sort of) we see that Riley has a mark that looks remarkably like an oversized vampire bite. Riley walks across the room, and we see that the whole attack had taken place within a few feet of three other men, without them noticing anything. He leads the other three back, and casually watches the blob get them, as well. Although they have been "taken over", they still retain a good bit of self-awareness. They don't start sounding like robots, and Riley sounds concerned for Kowalski's well-being. Kowalski leads Chip Morton off to be dealt with, and presumably the two other nameless men are engaged in similar pursuits.
Given orders by Crane, Riley elects to ignore them, which naturally arouses the Captain's wrath. Morton, fresh from his bout with the blob, suggests that he be sent to Sickbay. Morton's "meaningful" tones when he spoke to Riley made me think that this was an excuse to get the Doctor taken over--but that had already been done. The speed at which the takeover is accomplished is incredible.
Meanwhile, Nelson had found that some of the growth had clung to the gadget removed from the probe. He investigates and gets some idea of its properties--but not all of them, sad to say.
Either Crane was starting to sense something odd was going on, or else it is his habit to sleep with his gun readily to hand. Unfortunately, it didn't do him much good. As befits one of the stars of the show, his takeover scene was a good bit more extensive.
Crane fought the takeover so hard that it almost killed him--which seemed to leave a mark of sorts. The first time I saw this episode, I would have sworn that Crane had just pretended to go along with things. He seems just a little more aware than the others, and has some concern for Nelson and Sharkey. This contrasts sharply with the others, especially Chip Morton, who couldn't care in the slightest how much Nelson and Sharkey might suffer.
Nelson had ordered the Seaview on an intercept course with the Huron. Crane instead set course for Santa Barbara. On their return, Nelson gave their E.T.A. as five minutes, but since the course had been changed, not to mention the speed, the timing should have been different from what Nelson had calculated. In fact, they should have had a little difficulty finding the Seaview.
Once Nelson and Sharkey are back on board, the action speeds up considerably. Decoyed back to the Missile Room (and forgetting his own orders for quarantine) Nelson and Sharkey narrowly miss getting engulfed. They hit the alarm, which is ignored by everyone in the Control Room. Oddly, while you can see the officers moving about, all the common crew are standing as if frozen. (Perhaps officers' privileges continue even when taken over by big, shaggy balloons.)
Back at the Control Room, Nelson and Sharkey finally discover what's going on. The blob apparently has the capacity to move itself very rapidly from one part of the ship to another. Crane announces the creature's plans to eventually take over the world (can't they ever just want to take over a little bit of it?) Nelson and Sharkey temporarily get the upper hand and escape. Crane's vestiges of compassion vanish in the blunt command, "Find them and kill them."
Those corridors are sure empty, considering that there are 125 men on board. Guess most of them were taking naps. Nelson and Sharkey eventually make their way to the lab. Nelson has concluded that the thing has a central nervous system, and decides that electricity might be the weapon of choice. As their pursuers get closer, they hastily set to work. Luckily for Nelson, someone left a diving suit in his lab closet, which insulates him nicely.
Finally breaking into the lab, Crane's compassion returns. He sounds genuinely sorry for what's to happen. Nelson claims that he's tired of fighting, and walks right into the creature's embrace. Sharkey almost leaves it too long--Nelson had told him to hit the button as soon as he was in contact, but he didn't. Nelson had to yell to him. Interestingly, only Chip Morton attempts to stop Sharkey--perhaps he was the only one looking that direction. The resulting fireworks are spectacular. Nelson is left with his clothing in shreds, and the crew are all down and out.
The episode ends with Crane visiting Nelson's cabin. For some reason, it seems peculiar to imagine Crane wandering the corridors of the Seaview, dressed in his pyjamas and robe. He's concerned that they might still be affected by the creature, but Nelson seems certain that they are safe.
Ummm...Nelson seems to have forgotten that other chunk of extraterrestrial life, growing on board the Huron....