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Guest Cast:

Reisner.....................David Opatoshu 
Wesley......................John Milford 
Julie Lyle..................Jill Ireland 
Pennell.....................Steve Ihnat

Wear your mittens--it's cold down here!--Lee Crane

At Ice Station T in the Antarctic, Pennell is at the radio, trying to contact the Seaview, but is unable to reach them. His wife enters the building--they are on their honeymoon, doing research on samples of plankton. She suggests turning on extra heaters. He returns to the radio as she sets up the heaters and gets into bed. With still no luck, he starts work on his report, speaking into a tape recorder. As he talks and she sleeps, the plankton samples start to foam and then to grow, escaping their containers and turning into an enormous plankton monster which attacks and kills Pennell's wife. He tries to help her but the creature is too much for him. As he is eaten, he cries out "The plankton! Anna! Anna!"

The Seaview is, in turn, trying to contact Ice Station T, but with no luck due to magnetic interference. Nelson talks to Dr. Reisner, who looks at some scientific data and says it is wrong. The woman working with him, Julie Lyle, says that the Pennells are closer to the answer--Reisner interrupts and says that he is in charge and he will decide what the right approach is. Reisner feels strongly about this project; he has spent 15 years of his life on it and believes that plankton can become a food source for millions. Crane enters with a man, Philip Wesley, who is not happy; he represents the company funding this study and is unsure of the results they are getting. Reisner says that it can't be easy to share a cabin with a scientist--"with a Nazi!" interjects Wesley. Lyle jumps into the discussion, says that Wesley didn't mean what he said, and takes him out of the room. Sadly, Reisner says that the fact that he lived and worked in Hitler's Germany will never be forgotten.

At enemy headquarters, they have lost contact with the Seaview; however, they have an agent on board who will activate a homing signal at the proper time. To rule the people, one must rule their stomachs; plankton may be the key.

As the Seaview dives, it is rocked by an explosion in the laboratory. Crane and a fire detail enter and bring Dr. Reisner out of the lab--he can't understand why the chemicals reacted as they did. Nelson says there will be no more experiments while the ship is submerged, and the doctor agrees.

The ship surfaces to clear its air supply. As they prepare to dive, Kowalski contacts Crane from the missile room; they have visitors. Wesley and Lyle are touring the missile room. When Kowalski reminds them that this area is off-limits, Wesley says "not to us," and they continue. Lyle is particularly interested in the two-man minisub, wondering if one person can operate it, and Kowalski describes how it is done. Crane arrives and tells them the area is restricted; when Wesley starts to talk about his company's financial contribution to this project, Crane says that the area is still off-limits, and if they are found there again they will be confined to their quarters. Crane is called to the radio shack, and, leaving, directs Kowalski to escort their passengers out.

At the radio shack, Sparks notes an intermittent signal and says there is still no word from Pennell. Crane goes to Nelson, who is talking to Reisner, and asks to speak to him alone. Reisner leaves. Crane tells Nelson about the signal, thinks it sounds like a homing signal.

At enemy headquarters, they are receiving a clear homing signal. Their plan has two objectives: first, to obtain the secret for synthesizing plankton, and second, to ensure that the Seaview's crew does not return with their knowledge. The double agent is to be rescued at Ice Station T.

Crane goes to see Nelson; they have had no luck finding the homing signal, and he supsects that one of the three visitors planted it on board. Reisner is standing outside the door; Wesley sees him, grabs him, and drags him into the room claiming Reisner was spying on Nelson and Crane. Reisner says he was not listening--he came to consult with Nelson and didn't want to interrupt Nelson and Crane's conversation; Wesley doesn't believe him, but Nelson asks Crane to take Wesley forward while he talks to Reisner. Reisner says that Wesley's interference is jeopardizing the project; Nelson responds that, unfortunately, the project cannot be completed without money, which Wesley's company supplies. But Reisner is alienating everyone, and Nelson wants to know why. This project is very important to Reisner--after the time he spent in Hitler's Germany, he wanted to save lives, not take them. Nelson says he can't do it alone, and he has to have patience; Reisner says he has no time for patience. There was a reactor accident in his laboratory; he received a massive dose of radiation and will be dead in six months.

On reaching Ice Station T, Nelson--along with Reisner, Lyle and Wesley--go to the Pennells' lab. The wall has been torn out and there is no sign of the Pennells. Nelson notifies the Seaview that they are going to look for the cause of the destruction. Wesley wonders about "all this slime" he sees; Reisner assumes it's plankton but will have to run some tests.

At enemy headquarters, they activate phase two of their plan.

A plane approaches and radios the Seaview; the pilot, talking to Crane, says all he knows is that he was sent to "pick up some egghead," but that he has his orders with him and Crane can see them when he lands. Crane is suspicious of this change in plans, particularly since the Seaview was never notified about it, and radios Nelson; Nelson also knows nothing about this and tells him to check it out somehow. Crane asks the pilot where he's from; when he says the Bronx, Crane asks if he knows a Colonel Washburn. When the pilot says he does--plays cards with him, in fact--Crane knows he's not for real; Colonel Washburn died six years ago. He orders the plane shot out of the sky. Unfortunately, they miss, and in the time it takes to try again the plane is able to make an attack on Ice Station T. Crane radios Nelson to take cover, which they do, and after the plane passes Nelson says they must get back to the Seaview. On the way out, Lyle trips, falls, screams, and becomes a little hysterical; Wesley punches her and carries her out. The Seaview fires on the plane again, and this time they don't miss.

On the Seaview, Reisner puts the samples from Ice Station T in the lab. Meanwhile, Wesley goes to see Lyle. He apologizes for hitting her, saying it was the only way to save her life. When she notices the ship is moving, she again becomes hysterical, insisting on seeing Crane; the ship's doctor gives her a sedative, saying it will knock her out for an hour or so.

In the lab, the samples start to foam and grow.

Nelson and Crane listen to the end of Pennell's tape; Nelson says whatever it was must have come upon them suddenly. He says that the samples they found were plankton, but of a form neither he nor Dr. Reisner had seen before. Just then, the ship is rocked by an explosion, sparks fly and smoke rises from panels. All main controls are damaged, emergency generators are their only source of power, and no computers are working. They are blind and dead in the water, but are maintaining trim, and Morton hopes they can continue to do so. As they wonder what could have caused this, all heads turn toward Reisner, who looks uncomfortable and starts to leave. Crane asks him to wait. He turns to Nelson and tells him they were booby-trapped; a bomb was set to go off as soon as they dropped below 400 feet. He shows him a cigarette lighter with a radio inside. Nelson calls Reisner over and asks to borrow his lighter; when he can't find it, Nelson shows him the altered lighter and asks if it is his. He denies it, but Nelson has Patterson take him to his cabin, under arrest.

The growing plankton breaks out of the lab.

Nelson and Crane are discussing ways to repair the damage when Wesley runs in, demanding to know what is going on. Nelson tells him to get out, and he leaves. The lights flicker and the ship rocks; they have lost their generator, and are losing trim. Kowalski and another crewman, Fox, head for the gyro room to adjust them manually, but are blocked by the plankton. Kowalski escapes, Fox does not. The crew is able to drive the creature back with CO2, but not for long. They are able to trap the creature between bulkhead doors. Reisner doesn't understand how this could have happened, and he and Nelson go to review Pennell's records once again.

Wesley goes to tell Lyle about the sabotage. She already knows about it, since it was she who planted the bomb. She says there is one way out for two people--the minisub--and if Wesley will follow her, the two of them will be the ones to survive.

The plankton creature has broken through two bulkhead doors already; one more bulkhead and it will reach the control room. It follows Crane and his party in the other direction, toward the missile room, and when they reach it they close the bulkhead, keeping it out and themselves in. He calls the admiral on the intercom, wondering if they can stop the creature from that end. Nelson says they are working on it. He and Reisner have not yet found any useful information in Pennell's records. Reisner says something must have been added to the plankton, but Pennell left no record of it. Nelson glances through a file, but it has nothing but some personal papers, including the Pennells' wedding license. He notices that Pennell's wife was named Karen, and yet on the tape he was calling for Anna, Anna--he must have been trying to say that the plankton was anaerobic. Reisner says it was not something added to the plankton, but something taken from it, that caused growth--heat made it grow! Nelson calls Crane; they can freeze the plankton to a temperature where it will be dormant again and then blow it out like ballast. Crane points out that they can't control the temperature of the ship with all the systems damaged, and Nelson says that someone will have to go into flood control and open the hatch manually so that the cold seawater can cause the plankton to become dormant. Crane doesn't like the idea--whoever does it will be killed--but Nelson says it's the only chance for the ship. In a little while, Crane calls Nelson--he has an idea--but the communication systems go out and he is cut off before he can tell him. Crane's idea is to go out in diving gear to open the hatch from the outside.

The plankton goes through another bulkhead. In the control room, Reisner volunteers to be the one to go into the flood control chamber, as he has the least to lose. Meanwhile, Crane exits the ship, telling Patterson to continue to try to get through to Nelson, and Lyle and Wesley enter the control room.

Patterson gets through to Nelson, telling him Crane's plan. Not paying a great deal of attention to what is being said, Lyle pulls a gun. Nelson says he was almost sure that she was the saboteur. As she and Wesley move toward the hatch on their way to the missile room, Nelson tells her not to open the hatch and why, but she doesn't believe him. As Lyle backs into the hatch, she shoots at Nelson. Wesley is about to follow when he sees the plankton and hears Lyle's scream. Reisner pulls Wesley back and closes the hatch--if had not, the creature would have made it into the control room and they'd all have been killed. They hear the rush of water; Crane succeeded, and the ordeal is over.

Finally, Crane brings Reisner his lighter, found by a crewman under a table in the ward room. Nelson takes out a cigarette and Reisner lights it.