The episode begins with Sharkey delivering a Top Secret message for the Admiral. It's odd that Crane shows only nominal interest in it, considering the extremely humdrum nature of their current assignment: taking a marine census. In other words, counting seals. Just the thing this high-priced, ultra-sophisticated submarine was designed for.
Crane moves to the nose to make visual observations while Kowalski and Morton are stuck counting blips on the sonar. Convenient to the plot, the duty watch of standing in the nose has been dispensed with, so Crane is all alone when he spots the mermaid swimming casually along. Of course she's gone by the time Chip responds to Crane's urgent call. We see one of the essential differences between Captain Crane and Admiral Nelson: Nelson would never have admitted to anyone that he thought he saw a mermaid, whereas Crane tells Chip immediately. It's a good scene; even facing away from Chip, Crane can feel his unbelieving stare and quickly decides that he must have been seeing things. After Chip walks away, Crane spots her again. I don't understand why Crane didn't order the underwater cameras set to record, unless at this point he was embarrassed to do so.
Crane next tells Nelson, who reacts as you would expect. Crane at this point has become sure that he saw what he thought he saw, and wants to investigate. Nelson flatly refuses. The audience can guess that his refusal has to do with the Top Secret message he received, but Nelson doesn't say so--which he would come to regret. There was no reason why he couldn't have hinted that he had a good reason for not investigating--or that they could come back and do so later. Crane is surprised (and perhaps a little hurt) that the normally open-minded Nelson is not a little curious. Believing that they have time to spare, he arranges with Chip and Sharkey to be let off the sub near an island. They will continue the census for another four hours and come back to pick him up, giving him time to investigate on his own. Sharkey brings up the matter of regulations--divers are not supposed to swim alone. Crane, of course, is accustomed to breaking those regulations, and anyway, it's "unofficial". We see a very nice, thorough shot of the Seaview breaking the surface. Chip clearly disapproves of Crane gallivanting off on his own; Crane is remarkably blithe about it--he's got some leave time due, anyway. Because of the need to match footage, Crane is not wearing a standard diving suit (the excuse is that the water is warm). It's an unexpected bonus for the ladies in the viewing audience that David Hedison looks great in a swim suit. Nice legs.
Nelson eventually comes looking for the Captain. Sharkey, in a remarkably silly statement, asks him to specify which Captain, earning him a well-deserved bit of sarcasm from Nelson. Sharkey hastily passes the buck to Chip. Nelson is understandably aggravated, but I was rather surprised that he and Chip both referred to Crane's obsession. At this point, I would hardly call Crane's interest an obsession. After all, if you were certain that you had seen a mermaid, wouldn't you want to try and get some proof? Nelson is even more annoyed when he learns that Crane took no radio with him. Now, that was a good point; Crane should have allowed for emergencies. Nelson is forced to tell Chip information that should have been given to Crane, as Crane is "off on one of his personal ventures." That was an odd statement; it made it sound as though Crane's in the habit of running off on his own.
Certain nuclear powers are attempting a new disarmament policy. Certain unfriendlys don't want this going through, so they have planted a large nuclear bomb off the California coast. (Earlier, they had mentioned the island as being "off the east coast" which made it sound as though they were in the Atlantic, but apparently not.) The bomb is a sophisticated type immune to standard detection devices--and they have to find it in three hours. This, of course, begs the question: why did Nelson wait so long to act on the information? He told Chip that he was to inform Crane "at this precise moment"--but why wait? Under the circumstances, you'd think that they'd need to get started ASAP.
In the meantime, Crane has been pursuing his mermaid, who looks as though she's enjoying the attention. Crane is distracted at one point by a very large shark--probably the only thing that would distract him at this moment. The mermaid appears to duck out of its way, but shortly afterwards is seen being towed along by it, as though it were a playful dolphin. Crane ditches his underwater scooter--perhaps he knew it would get stuck in all that seaweed--and continues, with the mermaid almost beckoning him on.
Seaview shows up two hours ahead of schedule, but Crane is already waiting--with his catch, carefully covered up so that it simply looks like a large fish. Sharkey and Chip think that it may be a dolphin--although it's ridiculous to think that Crane would capture a dolphin. Twenty minutes after coming on board, Crane still hasn't reported to Nelson--he's in the lab with his catch. One wonders just what passed through Nelson's mind at this point--he clearly has an "oh, no!" expression on his face. Kowalski has been set on guard at the lab door. Sharkey mentions that the deck crew who assisted Crane on board think that his "fish" is actually a mermaid. When Nelson arrives, Crane immediately opens the door, looking shyly pleased. He points to the holding tank in the lab. The mermaid is lying senseless within. Nelson is flabbergasted, but he has no time to dwell on the matter. He hands Crane the Top Secret message. Crane glances at it, and hands it back indifferently. He's far more interested in his mermaid, whom he captured with a tranquilizer. (Crane himself sounds as though he took a good knock of that tranquilizer himself.) The mermaid had apparently led him to a grotto where a whole colony of them lived. Crane's concerned about the effect of the tranquilizer. It would have been more sensible of him to worry that this underwater creature might be slowly suffocating in that dry tank--or at least becoming dehydrated. Nelson slaps Crane--which Crane doesn't even seem to feel. Giving up, Nelson leaves, cautioning Kowalski to let no one in--or out.
Crewman Ron (who's called Thompson for this episode) is in the room that has the open hole in it. I've never understood the mechanics of this--why doesn't the water come up and flood the place? Perhaps in future they will consider putting a lid on it, because there's nothing to stop an ugly green something from popping up and climbing inside. Just what it is is a good question. Is it a guard dog of sorts for the mermaid--or is it actually the male of her species? (If so, I can't really blame her for her coy games with Crane.) Coming inside, it immediately commences howling. Thompson yells for help--with the most authentic note of terror I've heard on this show. Nelson comes on the run--but is too late to keep Thompson from being thrown down the hole. That was a shock--they don't normally kill off the more familiar faces. Nelson has quite a struggle with the thing before it knocks him out and exits the room, still howling.
Sharkey finds Nelson, who wants the thing taken alive. He doesn't go into much detail, leaving Sharkey to naturally ask what he's talking about. Nelson makes the reasonable guess that the creature is on board because of the mermaid. Sharkey corrals two crewman, neither of whom sound like regulation Seaview crew. One of them speaks rather insolently to Sharkey, and they both act--well, rather stupid. The creature follows them to the Missile Room. Stupid or not, they fight pretty well--although you can clearly see the wires on the one crewman who gets lifted and thrown by the creature. Sharkey himself gets thrown into the panel at the side of the escape hatch--which lets out a burst of sparks. It's hard to believe that he wasn't killed. The fight sequence was quite extensive--they kept knocking the creature down, it kept getting up. It finally got the three men down long enough to escape the room. Despite being belligerent and noisy, the creature also seems fairly intelligent--it enters the Circuitry Room after taking a close look at it. (Do they read English?) Nelson returns to the lab, where Crane is still standing, enthralled. The creature starts ripping up the Circuitry Room, and, in the lab, the lights dim. They did a rather clumsy job of editing at this point--the mermaid awakens and looks around, while Crane, who is looking right in the tank, never notices. The mermaid quickly pretends to be asleep, giving a quick smile of satisfaction. Presumably she knows that the creature--pet, boyfriend, husband, father?--is on board. Seaview lurches. Crane, whom we've seen dozens of times being pitched back and forth in the Control Room without even a bruise to show for it, tips his head back a bit, and is promptly knocked cold. The mermaid peeks around again. The creature is still at it in the Circuitry Room--and still howling. Seaview loses power and goes down--scraping against our old friend the rock outcropping. The mermaid takes yet another peek, while Nelson attempts to rouse Crane, without sucess. Nelson leaves, ordering Kowalski to get Crane to Sickbay--but the door slams and locks in his face. Apparently psychokenesis is a mermaidenly talent. The mermaid peeks around yet again.
Chip Morton has countermanded Nelson's order for Sharkey to search for the creature--they urgently need to make repairs. Nelson points out that catching the creature is even more urgent. Sharkey has gone to the Circuitry Room door--which slams and locks in his face. He goes to report--startling the heck out of Kowalski, who actually yells in reaction--very rare on this ship. Reporting to Nelson, it doesn't occur to anyone that, first, Crane might have gotten up and slammed the lab door, and, second, the creature might have slammed the Circuitry Room door. They're convinced that the doors shut of themselves. Sharkey is sent to collect stun guns, and they head back to the Circuitry Room. The creature is still in there, and they take it out with 3 shots.
Crane awakens in the lab. The mermaid is watching him, but Crane is no longer enthralled. The knock on the head (such as it was) seems to have restored his senses. Crane thinks that she somehow influenced his mind. Quite a trick while she was unconcious--and if she had such influence, why didn't she prevent him from tranquilizing her? (Maybe she was trying to make tall, green and noisy jealous. If so, it worked. Boy, did it work.) Crane acknowledges that he made a mistake in capturing her, and he doesn't want his crew to have to pay for that mistake. Crane also shows that his mind wasn't so far off in limbo as it seemed--he remembers what Nelson gave him to read, and offers the mermaid a bargain--he'll set her free if she'll look for the bomb. After all, if it goes off, her people will also be killed by it. I don't know if it was a matter of direction or acting ability, but the mermaid's expression remains more or less the same throughout this entire sequence--she just stares gravely at Crane.
With the creature confined, Nelson gets the circuits repaired, and Seaview lifts off the bottom. Going back to the lab, they find the door unlocked. Nelson slips inside--he doesn't seem to want anyone else to spot that mermaid. Both she and Crane are gone. Crane has taken her to the open hole, and after a long gaze into each others' eyes, she slips away. (I had visions of her bumping along the bottom of the hull as it moved forward.) Meanwhile, a crewman has gone to look at the creature, which of course promptly breaks free and kills him. Crane hears the howling, and goes to investigate. The creature bursts out, grabs him, and carries him off. Crane looks ridiculously like an ingenue in a cheap monster flick ("unhand me, you brute!") Did the creature somehow recognize Crane as the one who captured the mermaid? If so, Crane's lucky that it didn't kill him--it just dropped him, hit him, and left. Crane, when found, simply says groggily, "It came through the wall"--assuming that the others know what he's talking about. The creature gets to the lab, to find the mermaid gone. It's still howling. If it weren't for the occasional moments of silence, you'd think it was incapable of keeping quiet. It smashes a few bottles, throws a few boxes and a crewman around, and calls it a day, heading back to the hole. No one actually sees it leave, but they take it for granted that it's gone.
Back to the business at hand. Crane is certain that his mermaid will search for him. Nelson, incredibly, tries to deny at first that there was a mermaid, to Crane's astonishment, then points out that Crane has no way of knowing if she actually understood him, or agreed to help. He trails off as he spots her outside the nose. Nelson's reaction--"O.K, you win" sounds very peculiar--it's as though he's disappointed.
Crane suits up, along with Kowalski and three other divers (including Ron, returned from the dead). The three are to somehow neutralize the bomb so that Crane and Kowalski can safely disarm it. The mermaid leads Crane to the bomb--and then vanishes. The creature, still jealous, heads for the three divers (you'd think that Crane would be its target). It howls underwater, too. After knocking the divers around, Chip Morton manages to hit it with a laser. (If the creature was the mermaid's boyfriend or mate, that could explain why we don't see her again--she must have been horrified.) The divers' equipment is smashed, which means that Crane and Kowalski must do it the hard way. Nelson talks them through it. The disarming is sucessful--and one must hope that the mines surrounding the thing were automatically disarmed as well, because Nelson had them just drop the detonator without considering where it might land.
Crane receives an official "well done" from the Office of Naval Intelligence, although he doesn't think he deserves it. Oddly, it's Crane who starts to think that he imagined the mermaid, and Nelson who assures him that he did not--although perhaps it's better that she remain a mystery.