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Guest Cast:

Rear Admiral Tobin..........Charles McGraw 
Chief.......................Adam Williams 
Air Force General...........Frank Ferguson

I always said mermaids make lousy housekeepers.--Lee Crane

The military is tracking a glowing, flying saucer; an Air Force general orders missiles prepared to fire on it, but hesitates. Admiral Tobin tells him not to wait too long; the general says he would feel better about attacking if he knew what it was he was firing on. He starts the countdown, and then the saucer drops into the ocean; he says to hold fire.

On the Seaview, the crew is preparing to takes seawater samples when a message arrives. They have a new assignment; Nelson doesn't know what it is, but they are to proceed immediately to a particular position--and for the time being they are back in the Navy! When they reach the specified coordinates, they rendezvous with a helicopter and take on a passenger--Rear Admiral Walter Tobin--or, as Crane calls him, "Trigger-Happy Tobin."

On board, Tobin briefs Nelson and Crane on the situation with the flying saucer. The Navy already sent two subs after it; the first found nothing, the second radioed that they detected metal, but were lost. The Seaview is to pick up where the second Navy sub left off. The Seventh fleet will be there as soon as possible and the Seaview will function as Tobin's flagship; he says that it is their job to "get it before it gets us." Speaking to the Seaview's crew, Tobin describes the mission, saying that they have to assume that the saucer is hostile. The Navy commissioned the Seaview because it is has more speed, maneuverability, and firepower than any other ship; they are to find the saucer and force it to surrender. They eventually make metal contact, and Tobin tells Crane to prepare torpedoes. They turn on their sail and nose cameras; first they see a great deal of torn-up vegetation and run into unusual turbulence, then they pick up a high-decibel sound and try to find its source. As they near the source, the waters calm, but they begin to receive radio and television interference, so they go to the observation nose. As Nelson describes the probable source of the sound, they see the saucer rise from below. Lights blink, and they hear rhythmic sounds; Nelson wonders if the saucer is trying to communicate with them.

The missiles are ready to be fired, and Tobin says to do so; Nelson, on the other hand, says he has not yet seen any act of aggression on the part of the saucer. Just then the saucer glows more brightly, the Seaview starts to shake, and then loses all power. All systems are out--they have no air. The ship starts to move, although it still has no power; Nelson says that some magnetic force is pulling the saucer and the Seaviewtoward to the surface--but they are moving very slowly, and it's uncertain whether they will reach the surface before they run out of air. One of the crew becomes claustrophobic and hysterical; the chief has to punch him to calm him. Crane describes the situation to the crew, saying it's very possible that they'll reach the surface before the oxygen is gone, and telling them to stay at their stations and conserve their energy--and breath.

Tobin is furious that Nelson didn't fire when he had the chance. Nelson doubts that the Seaview could have damaged the saucer, and points out that the saucer's actions may well have been worse if the Seaview had been the aggressor. Just then, the ship shakes and comes to a stop; Crane estimates their depth to be about 400 or 500 feet. Out of the window, they see something--a small saucer leaving the large one. They don't know what it is--Tobin thinks it looks like a weapon--but it settles on the Seaview's hull, over the hatch. He, Crane, Tobin and Kowalski go to open the hatch; there is now an airlock over it, which, Tobin says, may be filled with gas. Considering the possibility, Nelson tells everyone to leave while he cracks the hatch; Tobin insists on staying, as do the others. Nelson opens the hatch; he belives it is an escape chamber, with room only for one passenger. He enters it over Tobin's objections, and it returns to the saucer with him.

Nelson, finding himself in futuristic surroundings, calls out and a man in a space suit welcomes him. The man removes his helmet--it's Nelson! He comes from another planet, and has been studying the Earth; finding that the Earth's people fear that which is different, he took on a form he judged most unlikely to offend Nelson. Nelson tells him he has more human traits than he realizes; the alien says these traits include fallibility--the saucer came to Earth by mistake. While in the Earth's solar system, they collided with a meteor and ruptured a fuel line, causing their crash into the ocean. He claims that they did not attack the Seaview, and that the effects of their ion ray would wear off in an hour or so; Nelson says that will be too late--the crew has no air. Unfortunately, the saucer is unable to surface, so cannot take the ship up. Nelson says that if the Seaview can get within 100 feet of the surface, they can use a manual breathing device, a snorkel, to get air; the alien says he will bring the sub to that depth. Nelson asks why the alien doesn't simply release the Seaview, but the alien is distrustful of Nelson and unwilling to take his word that they will not attack the saucer. He is, however, pleased to learn that Nelson is a scientist.

Meanwhile, the Air Force is scanning the ocean's surface; they detect the Seaview but can get no response from them. The general assumes the sub is in trouble--if they could answer, they would.

On the Seaview things are getting uncomfortable. The alien causes the ship to rise, and Crane orders preparations for surfacing; however, they are disappointed to stop again at periscope level. Crane realizes that this means they can use the snorkel for air, and soon the crew can breathe again, an immense relief.Crane looks through the periscope and sees jets in the sky; he realizes they are going to bomb the saucer, and with it the Seaview. They do, and aboard the alien saucer the alien is very angry with Nelson, believing that he lied when he promised not to attack. Nelson tells him that it wasn't the Seaview attacking, and if he would contact the attackers they would be willing to negotiate. Unfortunately, the saucer's communication system is incompatible with radio and no contact is possible. The alien has called home for more fuel; the ship that will bring it is similar to this one, but more heavily armed. Conflict seems inevitable--the Earth will certainly consider the arrival of the second ship to be an invasion, and the Seaview will undoubtedly attack the saucer as soon as the ion ray wears off, as the saucer hasn't enough fuel to use it a second time. Another bomber attack will destroy the Seaview, says Nelson; the alien replies that it will also bring a retaliation that will destroy the entire planet.

Tobin wonders why the military isn't attacking, and Crane wonders why he is so eager for an attack that will destroy the Seaview as well; Tobin replies that his orders are to destroy the saucer, and that they won't be the first to die following orders.

Nelson and the alien have been touring the saucer. Nelson notes that it uses nuclear power, as does the Seaview; the Seaview uses uranium, useless to the alien saucer. Nelson says that when they want additional power, they also use pellets of strontium 90. The alien is excited; he says that this power source would be primitive but could be converted into a form they could use. With it they could get the saucer out of the Earth's atmosphere to rendezvous with the rescue ship, thus averting certain destruction of the planet. Nelson returns to the Seaview; once he leaves, the alien returns to his natural appearance.

On the Seaview, Crane has gone to get the strontium pellets, while Nelson and Tobin argue. Tobin doesn't believe the aliens will let Nelson return once they have what they need. He still wants to destroy the saucer as soon as power returns. Nelson insists that the aliens must leave the Earth as friends. Shortly after Crane arrives, the power returns; as Nelson starts to leave, Tobin tells Crane he is assuming command, and to arrest Nelson. Crane tells Nelson he is under arrest--then quickly leaves the room, taking the strontium and locking Nelson and Tobin inside.

Crane takes the strontium pellets to the saucer; this time the alien resembles Crane. He thanks Crane and sends him back to the Seaview. On the Seaview, the Chief lets Nelson and Tobin out of Nelson's quarters. In the control room, Morton notes the arrival of the Air Force. They are about to attack when the Seaview contacts them, telling them the captain is still on board the saucer. They are given 10 minutes to get him back and get out of range. Crane soon returns. The ocean starts to bubble and the saucer rises, out of the sea and into the atmosphere, and out. Tobin says that maybe it's just as well. Crane says that the aliens will be back--will they remember they were treated as friends? Nelson replies that they will--but will we?