It begins in a frozen waste, with one lonely building out in the middle of nowhere. Ice Station T-For-Ticktock. A man, clearly a scientist, is studying an array of beakers before moving to the radio and attempting, without sucess, to call the Seaview. We will learn that his name is Pennell. His wife comes in from the cold (it's never explained what she was doing out there in 45 below). Although her husband is presumably comfortable enough, she pulls out some extra heaters and turns them on. Some gentle bantering makes it clear that the couple is on their honeymoon, as well as engaging in research. Mrs. Pennell opts for a nap. (One might question bunk beds on a honeymoon, but remember, this was the early sixties.) Giving up on the radio, Dr. Pennell begins recording his observations on plankton, mentioning photosynthesis and the action of the plankton rising and sinking in the water according to the amount of available light. Behind him, unheard, the beakers begin to bubble, and soon...things...start extending outward (in a really nifty effect). Apparently Dr. Pennell is the sort who keeps quite still as he concentrates; a fair amount of time must have gone by and yet he never turned in his chair to notice the strange growth, which has swollen to a huge, spiky mass. It's not until the thing knocks over a heater that it is noticed. Mrs. P, waking to a living nightmare, reacts in tried-and-true ingenue fashion by screaming her lovely head off. Dr. P. struggles frantically to reach her, but is quickly overwhelmed. Knowing that the recorder is still on, he desperately tries to make the situation known, but he is rapidly becoming smothered. He manages to get out "The plankton!" before gasping, "Anna...Anna...." This scene would not have been nearly as effective if it had been Dr. Pennell alone, or if his assistant had been another man. In just a few minutes, with some brief dialogue, we have come to like and admire the Pennells, just beginning their lives together, and their deaths are that much more horrifying and unfair.
On board the Seaview, we learn why Dr. Pennell had been unable to get through on the radio--a storm is causing extensive magnetic interference. Down in the lab, we learn that Seaview has guests aboard, apparently relating to the work being done by the Pennells. Dr. Reisner has just concluded that his current experiment had taken the wrong approach, but takes exception to Julie Lyle's comment that she already knew that it was wrong. Julie is apparently a government-employed scientist, while Reisner is private. The whole situation is a joint venture of private industry and the Nelson Institute, and apparently relates to using plankton as a food source. Nelson points out that they should reach the ice station within 24 hours, but Dr. Reisner, in an arrogant breach of protocol, calls the radioshack directly and demands information, annoying Nelson considerably. The irritation is quickly added to when Crane enters the lab with a third guest, Philip Wesley, who represents the "private industry" that is financing the project. Wesley has weight to throw around and he does so with gusto, and ends his rant by calling Reisner a Nazi. Julie hastily drags Wesley out of the room, while Reisner, deeply offended, retreats to his quarters. Reisner had, in fact, worked in Germany under the Nazis, and apparently no one is going to let him forget it. Crane notes that Nelson does not like Reisner, but, perhaps wisely, does not ask if Reisner's background has to do with the dislike. Nelson is willing to put up with Reisner because he is the best man for the job. Crane's job is even trickier--trying to keep the journey on an even keel in the face of rapidly increasing tensions.
The scene cuts to a fancy manor house, and a view of the bad guys. They seem to be living the high life--surrounded by elegant furnishings and artwork, fine wines, and the latest in electronic equipment. They are keeping track of the Seaview. Although the magnetic storm is causing interference, they have no fear of losing contact--they have an agent on board.
Crane is checking on the ship's gyros when they are rocked by an explosion from the lab. With swift efficiency (an absolute necessity with a submarine) he calls in a fire detail, drags Dr. Reisner to safety, calls for a medic, and orders the Seaview to the surface to be scrubbed clear of the smoke. (I've always wondered what's involved in "scrubbing smoke".) It almost looks like an act of sabotage--Dr. Reisner is bewildered at the explosion of his "simple compound"--but this line is never followed up. Crane simply orders that no further experiments take place while the ship is submerged. Having been scrubbed, we are given a full diving sequence. Shortly afterwards, Kowalski requests Crane's presence in the Missile Room, as they have visitors. Julie and Wesley, blithely ignoring Kowalski's statements about restrictions (rules can't possibly apply to them!) make themselves at home. Julie is very interested in the workings of the mini-sub. As it is not a classified piece of equipment, Kowalski unbends enough to explain how it works. Crane arrives and icily points out the "Restricted" sign on the door. Wesley attempts to throw his weight around again, but Crane puts on his Captain's hat and exerts his own authority. Crane is then called by Sparks in the radioshack.
Dr. Reisner is conferring with Nelson. He fumbles out a cigarette, then starts patting his pockets before Nelson strikes a match for him. Crane enters the room--he needs to speak with Nelson privately. Reisner retreats without argument. Crane reports that they are picking up a strange signal, right on top of them--as though a submarine was riding them piggyback. It sounds like a homing signal. Back at the manor house, the bad guys clarify their plans: First, obtain the secret of the plankton, second, destroy the Seaview--after their agent has been rescued, of course. Meanwhile, Crane has been trying to trace the source of the signal--he thinks that someone brought something on board, and they have three strangers that make dandy suspects. Dr. Reisner, hesitating outside the door at the sound of voices, is spotted by Wesley and shoved into the room, accused of eavesdropping. Reisner protests that he simply wanted to talk with Nelson, but hadn't wanted to disturb him, and counterclaims that Wesley is jeopardizing the project. Nelson points out that money is a vital necessity, and they have to put up with the resulting aggravations. Reisner, on the other hand, has alientated virtually everyone on board. Reisner's excuse is that he has no time for patience and courtesy--he has only six months, thanks to a radioactive accident. He also makes clear that his staying in Nazi Germany had been a matter of misjudgement.
We're treated to the broaching shot of the Seaview, once they find some open space in the ice. The three guests and Nelson get bundled up for the ice station. They find the building broken open. The inside is a shambles, the Pennells are nowhere to be seen, and there are slimy bits of plankton everywhere. They begin collecting samples, while Nelson finds the tape recording. The bad guys learn of the Seaview's arrival, and set their next phase in motion. An aircraft, identified as USAF JKS 149, merrily states that it has been sent to pick up a member of the science team. Crane agrees to its arrival, but quickly has second thoughts. Nelson has no knowledge of any such pick-up. Calling the plane, Crane casually asks where "Smilin' Jack" comes from--the Bronx--and asks if he knows Col. Washburn from the same area. "Smilin' Jack" falls for the trick question--Washburn has been dead for six years. Crane immediately orders the plane destroyed. Because of continued magnetic interference, the missile misses. The plane goes to attack mode, firing into the ruined ice station (surely very dangerous for the agent inside?) The team makes a run for the submarine--all except Julie, who jumps to the side, crouches down, and promptly calls attention to herself by screaming. Quite possibly they wouldn't have noticed her gone until they reached the Seaview, but Nelson naturally turns back and urges her to come along. Having no time to deal with hysterical women, Wesley does what he has to by dealing her a crisp punch to the jaw. He and Nelson carry her off. By this time, Seaview has managed to calibrate and launch a missile manually, and this one hits dead on. Julie is lowered into the Control Room via a rope harness, and Nelson, for some reason, orders her sent to his cabin. Why not her own? As the only woman on board, she would surely rate private quarters. Dr. Reisner prepares the samples of plankton, then leaves the lab. A young man, presumably the ship's doctor--although he may simply be a medic--examines Julie. He can't tell if she's suffering concussion (good evidence for thinking that he's not a doctor). Julie becomes frantic when she realizes that they are underway, and the man prepares to sedate her. (More evidence--it is not a good idea to sedate someone with possible concussion.) Julie starts screaming for Crane, but the medic and Wesley, complacently certain that she's simply hysterical again, hold her still and drug her senseless. I'd like to know just what she would have told Crane if she could have reached him. The identity of the agent is now pretty well obvious.
In the lab, ominously, the beakers are bubbling. In his cabin (Julie's not there, so presumably Nelson's order was ignored) Nelson and Crane listen to the end of the tape--and we learn that Dr. Pennell's final cry of "Anna" was superceded by a ghastly scream. Nelson apparently did not clue in to Pennell's cry of "The plankton!" even with the evidence of plankton all over the ice station. The Seaview is rocked by yet another explosion. Crane seems to make a standing leap from Nelson's cabin to the Control Room--he's already there when Nelson and Reisner show up. Crane stares at Reisner with a very obvious expression on his face, and Reisner quietly moves out of earshot. The Seaview had been booby-trapped with a mini limpet rigged to their depth gauge--it went off at 400 ft. They have also located a cigarette lighter with a hidden antenna. They guess, accurately, that the saboteur meant to get away in the plane. Nelson, perhaps recalling how he lit Reisner's cigarette, asks him for the loan of his lighter. Reisner is happy to do so--but he can't find it. Faced with a lighter that he first identifies as his own, Reisner can make no adequate explanation and is taken by Patterson to his cabin. (Considering that he shares a cabin with Wesley, it seems odd that they didn't simply take him to the brig. Perhaps they were aware that the evidence, while good enough for the moment, was not actually very strong.) Wesley, very belatedly, comes in to the Control Room, wanting to know what's going on. Crane and Nelson, both aggravated, chase Wesley out. The Seaview starts to lose trim, and Kowalski and Crewman Fox are sent to the Gyro Room to check. Fox is taken aback at finding a huge, spiky mass wrapped around the gyro control, but he gamely plunges in--realizing too late that this was not a good idea. Kowalski tries to drag him out, Fox beginning to cry out in pain. (Perhaps the plankton's unchecked growth is why it is seeking nourishment from protein rather than photosynthesis, but there is no actual attempt to explain why this plant suddenly turned carnivorous.) Fox's hand slips away. Kowalski slams the hatches shut and takes off. Back in the Control Room, Curley only needs a look at 'Ski's face to know that something is horribly wrong. Del Monroe's horrorstruck reaction, including his frantic bleat of "Fox!" is superb. His explanation is far from clear, but it's obvious that they aren't going to get anything more from him at present, and Nelson, Crane, and Curley run for the Gyro Room. They're brought up short at the sight (there's not a trace left of the hapless Fox). Curley is sent for CO2 tanks. Nelson realizes now what happened to the Pennells. The mass reacts to the CO2. More crewmen rush up, to stand in horror. One is sent to fetch Dr. Reisner, while Nelson tries to fix the gyro. The mass starts expanding back into the room, forcing Nelson to duck out one door, while Crane, Curley, and Patterson duck out another (Crane nearly getting munched in the process). Nelson yells to them to dog all hatches as they go--they will try to contain the mass amidships. Nelson then hauls Reisner off for another check of the Pennells' records.
Wesley, who was chased out of the Control Room without being told anything, has somehow found out anyway. He bursts into Julie's room (where she has awakened from the sedative) and announces that they have been sabotaged and are all going to die. Here is a situation where throwing his weight around isn't going to accomplish anything, and he's in a panic. Julie, who has recovered from her own panic, admits casually that she is the saboteur, and plans to escape--and she's willing to bring Wesley along. The mini-sub works best with two. Wesley, concerned only with his own skin, doesn't even react to the news of her treachery. Crewman are racing like mad through the corridors. Crane pulls up as Chip Morton calls over the intercom. The mass has burst through two bulkheads already, and is moving into the Flood Control chamber, right next to the Control Room--it's only a bulkhead away. We can get a guess at the sheer size of the thing by the fact that it is also rapidly closing on Crane and Company. Crane stares at it, frozen, and is shoved along by Curley. They end up trapped in the Missile Room. Reisner, meanwhile, is certain that something was done to the plankton to make it grow--a reasonable deduction. Unfortunately, Pennell left no record of anything that he did to the plankton during his experiments. Nelson glances through a folder of personal papers, including the Pennells' marriage license. Nelson suddenly notes that Mrs. Pennell's first name is Karen--so why had Pennell died calling for Anna? He realizes that the word Pennell was trying to get out must have been "Anaerobic". I'm not sure how they managed to jump from this deduction to "Anaerobic Bacteria", but they do so, and conclude that the bacteria was what kept the plankton's growth in check. Heat destroyed the inhibitor. (This is another deduction that seems to come out of thin air--perhaps Nelson had noted the extra heaters in the ice station.) They believe that cold will destroy the thing. (This is actually an unwarranted assumption--the plankton ordinarily lives in the cold waters. Cold should only stop it from growing any larger; it shouldn't kill it.) Because of the damage already inflicted, they cannot remotely flood the corridors with icy sea water. Someone will need to go inside to open the hatches--which is a fair way of committing suicide. Nelson relays this to Crane, who has an idea--but the intercom gets cut off before he can do more than say "Don't--". Seaview is starting to sink. Crane suits up to go outside--he can only last ten minutes in the cold, and they will have to be enough. The mass breaks through the next bulkhead--it is now right next to the Control Room.
Reisner, having nothing to lose, offers to go in, although how he or anyone else could hope to accomplish anything before being engulfed is a good question. At this auspicious moment, Julie and Wesley stroll into the Control Room. Somehow, they've completely missed the news of the more serious threat. Patterson manages to fix the intercom. As they learn what Crane is doing, Julie pulls out a gun and silently motions the crew back. Nelson had just about decided that Julie was "it"--probably her attempt to stay behind at the ice station clued him in, although he doesn't specify. They attempt to warn Julie and Wesley of the danger, but saying that the corridor is "full of plankton" just doesn't convey it. Wesley opens the door. Reisner jumps forward, and Nelson grabs him--and gets shot for his pains. Julie, in what I confess is a very satisfying moment, gets eaten. Reisner yanks Wesley away from the door (Wesley, true to his nature, stands frozen as Reisner wrestles the door shut). Moments later, they hear water coming in from outside. The plankton is doomed.
Sometime later, with the Seaview patched up and on her way, Crane comes to Reisner and quietly hands over his lighter, which a crewman had found in the wardroom. Neither he nor Nelson offer an apology, but Reisner, magnanimously, is not concerned with one. He wonders if the knowledge gained was worth the price they paid for it--which presumably accounts for the episode's rather odd title. Nelson is certain that it was worth it--even if they don't see the results in their lifetime. They head for home, with Reisner's 15 year project presumably shelved for the time being.