They never say exactly why the Seaview happened to be there. One might infer that they were investigating stories about the area; or was it a coincidence that Nelson happened to be looking through an antique book that had stories about the place? 300 years previously, a British fleet had gone down in a storm--but not before hearing a "rock-like symphony" sounding from the depths.
Patterson picks up strange noises on the hydrophones. Shortly afterwards, sonar picks up strange readings, and then they are abruptly hit with turbulence, causing enough damage to require three hours' repair. They hear a heavy tapping sound coming from the nose. Opening the collision screens, they find what appears to be a humanoid shaped mass of rock. (Seaview is not on the bottom; one can only conclude that these rocks can swim.) Nelson and Crane are left groping for explanations. Crane orders out a diving party.
The party will be two: Kowalski and a newly certified diver, Richards. Kowalski, concerned for his friend's inexperience, wants to know what they're getting into. Rather than admit that he hasn't been told, either, Sharkey stands on his rank and tells them the Captain will tell them what they need to know. Crane arrives, rather taken aback at seeing the new diver, but Kowalski assures him of his competence. Crane orders them to check on the source of the turbulence--plus any strange shaped rocks they might see. The divers are not out very long before Richards is caught up in another burst of turbulence.
Inside, the hydrophones are picking up sounds again. Crane this time anticipates more turbulence to follow--and he's right. Oddly, they did not notice the previous turbulence, unless Richards got caught up in the same surge that tumbled the Seaview. We see a rarity--Patterson actually gets injured in the rock 'n roll, although he brushes it off. The sounds suddenly stop. A fire in the Circuitry Room leads to the main panel being damaged. Another two hours' worth of repair. Meanwhile, Kowalski has returned alone, having lost track of Richards. Nelson orders another diving party, while he goes out in the Flying Sub. 'Ski, understandably upset, wants to go back out himself, but Crane orders him back to regular duty. (Under the circumstances, shouldn't he have been given a little break time?) The divers find Richards, apparently unconcious. Nelson and Sharkey remain out in FS1, now searching for rock shapes and such. They begin hearing sounds, and follow them into a fissure that opens out into a cave.
In Sickbay, Richards is delirious. Doc says that he seems to have been attacked by something. Richards is clutching an odd little lump that looks like a fossil. Meanwhile, a heavy current has grabbed the FS1, and Seaview loses contact. Chip Morton reports that signs indicate that the Flying Sub might have exploded.
As usual, the radio in the FS1 has gone out (they really need to come up with a sturdier model). Finding themselves in an underwater grotto (and having presumably checked to see that there's breathable air) Nelson and Sharkey go exploring. Sharkey finds a 300-year-old flintlock and manages to fire it. Must have made pretty potent gunpowder 300 years ago. Sharkey's lucky it didn't blow up in his hand. The shot awakens a figure embedded in the cave wall. (Was he snoozing there long enough to get sealed in, or did he mortar himself in, akin to tucking up with a cozy blanket?) Attempting to leave, they are quickly surrounded by three other walking piles of rock.
Back in Sickbay, Kowalski attempts to talk with his friend, but Richards is quite agitated and yells at him to leave. Checking on him after 'Ski leaves, Doc finds that Richard's right arm has become dark, with a texture like fine pebbles. Richards jumps up and flattens him.
Back in the grotto (the scenes jump back and forth quite briskly) a fourth fossil man approaches, signalling the others away by clicking with his...fingers? Claws? Whatever. This man is able to speak (although he doesn't have a mouth as such). He claims to be Captain Jacob Wren of the submarine Troilus. Nelson is startled; the Troilus went down 30 years ago. Wren owes his life to the fossil men. They were apparently once ordinary men, thousands of years ago, before becoming trapped somehow in this area. A strange combination of elements caused their bodies to mutate into rock form. Wren himself is still in the process of changing. He invites Nelson and Sharkey to join them and change as well--but they would just as soon not, thank you very much. Wren claims that they simply questioned Richards when they had him--a claim that the viewers already know is a lie. While talking, another rock man, this one with a recognizable head and face passes through. A ritual is about to begin; Wren invites Nelson and Sharkey to come and learn more. I would like to know more about this new rock man. Where did he come from? We've not heard of any recent disappearances in the area; is the man a member of Wren's own crew? The man, in fact, looks a little leery of what is to happen, and it appears as though two of the others pushed him into the lava pit, rather than him jumping himself. Wren tells them that repeated immersions in the pit "wash out" the human impurities until they become fully fossilized. The man emerges from the pit--he now has a suitably lumpy-looking head. Wren also claims that the fossilization process is a "fountain of youth" that frees them from human ills and aging. I don't know that immortality would be worth looking like a disreputable cousin of Ben Grimm from the Fantastic Four, and Nelson and Sharkey would seem to agree. Nelson seems certain that Wren intends to fossilize them whether they want it or not, and why? Wren informs him that he wants to use the Seaview to transport his men (apparently they can't swim very far) to the "promised land"--the world above, where they will take it over, using the Seaview's missiles. Nelson protests that firing procedures are top secret--but Wren already knows, from Richards, that the procedure is locked in Nelson's cabin. Once again, Failsafe is tossed by the wayside--not to mention the idea of a common seaman being privy to such information.
On Seaview, Richards has made his way to Nelson's cabin. His walk is very effective--he looks as though the weight of his arm is pulling him off balance. He paws awkwardly through Nelson's desk before smashing open the safe. He finds the relevant information and tucks it away just as Crewman Ron attacks from behind. Ron should have signalled for help before diving in; Richards deals with him almost effortlessly. Going to the Circuitry Room, he quickly chases out Kowalski and another crewman, then commences tearing up the place. Seaview lurches accordingly. Kowalski, badly shaken, comes on the run to report. Somehow, he failed to notice Richards' arm. Sparks gets the rare treat of taking over the con while Crane and Morton both head to the Circuitry Room.
Wren tells Nelson that he plans to rule the world. Did his fossilization drive him mad, or did he always have such ambitions? And why would these men, who presumably are much, much older than he is, give him leadership? Has he delayed his complete fossilization in order to retain the ability to speak? That would make sense, because I can't see how he could expect to rule the world if he could only communicate with clicks that humans wouldn't comprehend anyway. Wren goes to assemble the "council" to supervise Nelson and Sharkey's first dip in the pit. It's absurd to think that a soft human body could survive even an instant in a pit of molten rock. It would be faintly plausible if they were first partially fossilized (like Richards' arm).
Coming into the Circuitry Room, they are stunned to find Richards almost completely fossilized. This is a really noticeable inconsistancy--Richards has not had immersions in their version of a hot tub, and yet he is suddenly as far along as Wren, who has been fossilizing for 30 years. It would have been just as shocking, and much more plausible, if all his visible skin had taken on that dark, fine-pebble texture. (And what happened to the Top Secret plans that had been tucked in Richards' waistband?) At this point, Richards seems to regain a little self-awareness; he seems stunned himself at what has happened. Chip, unfortunately, jumps the gun, taking hold of Richards and provoking an instant reaction. Kowalski attempts to calm him, and Richards, with the last dregs of his humanity, gasps at him to get away while he can. Locking the room, Crane sends Kowalski for a portable force field. Richards starts punching through the wall. (In a quick peek back at the grotto, Nelson and Sharkey start manuevering around their guard.) The force field knocks Richards back into the room, but not down. Crane decides to use the sonic machine--which is still in the experimental stage and very dangerous. They had a similar weapon in the episode "The Death Ship", which was used once and never seen again. I love these fancy weapons which pop up when it's convenient--which usually happens when Nelson isn't available to cobble something together.
Nelson and Sharkey knock their guard into the pit--which presumably will just make him bigger and lumpier. Running to the boat, Sharkey is temporarily brought down by another fossil man buried in the sand. (They do nap in odd places.) The fossil men start wading out after them, and end up standing on the FS1's diving planes. They manage to shake them loose, and get out of there. (Pretty neat trick, considering that they needed some repairs, which they didn't get to before heading out to explore.)
Chip Morton is very anxious about the sonic gun--it could blow out the side of the Seaview. Crane, on the other hand, seems rather nonchalant about it. He experiments using the fossil lump he got from Richards. He not only manages to blow up the lump, but also the tool chest it was sitting on, and the table the tool chest was sitting on. Seems effective enough. Richards manages to overcome the force field and get past Kowalski--but the sonic gun knocks him flat. They can't tell if he's dead or not. They lock him in a storage room, which seems pretty pointless given how easily he got out of the last room he was locked into.
Nelson and Sharkey return--and the noises start up again immediately. They rush to see if the sounds are coming from Richards. They are not. Kowalski has very imprudently put down the sonic gun in order to track the sounds. Nelson fears that Wren and his men will enter the escape hatch, and --ta dah!--they already have. They must give off a lot of internal heat (reasonable enough for men who bathe in lava) because they're completely dry after their swim. Richards bursts out of the storage room and blocks Kowalski from the gun. Wren makes one final offer for them to surrender peacefully and become nice little rocks. Nelson formally refuses. He and Sharkey suddenly dive past Richards, and Sharkey grabs the gun. One fossil man down. (You'd think there would be chunks of rock and pebbles flying like shrapnel.) Wren and Richards duck back into the Missile Room, leaving a very large fossil man to face them. One wonders if these fully fossilized men lose any awareness of danger. Or perhaps their brains fossilize as well. Another fossil man down. Wren and Richards set the missiles to hit the United States (guess Richards must have managed to pull those instructions out from under the rocks). Sharkey runs for some explosive,Kowalski hides with the gun, and Crane takes to the air ducts, while Nelson attempts to reason with Wren. Wren will accept nothing more than unconditional surrender, and tells Nelson to come inside to get his orders. Nelson agrees and comes inside, seeing that Crane has already made it through the ducts. Wren orders Nelson to take Richards to the Control Room to take over, ignoring the fact that Richards has no training in such matters. Coming out, Nelson yells for Kowalski. Irwin Allen might not have appreciated personal dramatics with his actors, but Del Monroe puts enough expression on his face that you can imagine his anguish at killing his friend, although that's as far as it goes. Wren shuts himself back in. Crane, meanwhile, slips past and starts frantically deactivating a missile in between dodging an angry pile of rock. At least he has the sense not to swing at it barehanded, but smacking Wren with a wrench wasn't much better. Nelson and the others manage to blow the door and get inside just in time to save Crane from getting crushed. The propulsion fuel from one of the missiles explodes (well, that's what you get when you're in a hurry). Wren seems not much damaged, and is still implacably set on conquest. Without waiting for an order, Kowalski shatters him. Nelson follows up by sealing the grotto with a well-placed missile. Quite a fun outing, which I didn't really expect with a bunch of walking rocks.