Damaged by an explosion from an old mine field, Seaview takes a dive to the ocean floor. Listing dangerously, running out of oxygen, having to deal with fire and flood, the crew can do nothing but wait for help. But will help arrive in time?

"Idiots!!!" Admiral Nelson to himself. There is an imposter on Seaview. Lee Crane is the main suspect. Admiral Nelson takes Morton and Sharkey into his confidence. They must put Crane to the test to find out whether or not he's the man they know. Captain Crane is really put through the ringer in this episode! Admiral Nelson hurts his feelings on purpose, the Captain practically chokes to death from a fire in the Circuitry Room that he gets blamed for, he gets locked in a storage room, and he's driven half out of his mind by the Admiral, Morton and Sharkey. Through it all, Crane maintains his inner belief in himself.

An alien spaceship lands in the ocean, and Seaview is sent to investigate. Although Seaview is powerless in the vicinity of their ship, the aliens seem friendly, and Nelson manages to open negotiations. The military authorities, however, have other ideas, and Seaview may be in as much danger from human interference as from the aliens.

As Admiral Nelson demonstrates a new tracking device for a visiting U.S. senator, it seems to show that Seaview is the only life left on earth.

When a mass missile launching by a foreign power sets the U.S. on nuclear alert, the Seaview goes into a fail-safe maneuver, but is the launch really what it appears to be, and does this actually mean it's "doomsday?"

In an attempt to control the planet's tides, Dr. Randolph Mason (a nut if ever there was one -- he is part scientist, part make-up artist!) has invented a powerful electromagnet and established a magnetic field around the moon. The only person who seems to recognize the obvious danger in this silly plan is Harriman Nelson. At first glance, it would appear that Nelson has killed Dr. Randolph. At second glance, it would appear that Captain Crane has betrayed the Admiral. On third glance, the moon is getting closer and closer to Earth, and will soon collide with our beautiful blue orb -- unless the Seaview can save the world. Again. * In this episode, we learn that Chief Sharkey attended Lincoln High in Coney Island, Brooklyn, New York. (There actually is a Lincoln High in Coney Island.

Seaview and an enemy sub battle to recover a superbomb from the ocean floor.

Nelson and Crane are dismayed when an egotistical, publicity-seeking scientist is given command of the Seaview to test a new pressure system which will allow the sub to explore uncharted depths.

Drs. Paul Ward and Peter Omir have discovered a powerful new element -- Subterranium 116. This element can produce limitless pure energy, but Omir may have another agenda.

"Those who see the Flying Dutchman never, never reach the shore." Investigating the loss of Navy ships, Nelson finds an ancient dagger in a cave on an uncharted island, and falls under a strange curse. Haunted by visions of skeletons and an anachronistic ship, he has trouble convincing the crew of his sanity. Meanwhile, Seaview is sailing in circles, and the visiting Commander Van Wyck seems to have disappeared. Guest star Warren Stevens also appeared in "The Saboteur" and "Deadly Invasion".

After Seaview's sister ship, Polidor, is lost on an experimental deep dive, Seaview continues with the tests. Unknown to the already stressed crew, one of the visiting psychologists aboard is an enemy agent who has released a fear-causing gas into the air system. As Seaview plunges into the depths a dangerous panic starts to spread.

Crane is brainwashed by enemy agents, and his new mission is to deactivate certain strategic undersea missiles and to assassinate Admiral Nelson!

Cybernetic genius Tabor Ulrich creates a perfect mechanical duplicte of Admiral Nelson. His plot: to make the crew of the Seaview they are fighting World War Three. In doing so, the sub will launch nuclear missles at the China, Russia and the USA. After the holocaust, Ulrich plans to take over and usher in "the age of the cyborg".

Things go wrong when Nelson and Crane test out a new automated system for Seaview.

Nelson and Crane go undercover and risk their lives in order to bring a Communist defector back to the United States. But the man is injured in the attempt, and his lover is left behind. He won't speak to the agent waiting for him on Seaview (actually a double agent) until he is reunited with his Anna. But even Anna is not all she seems.

Injured in a fall, the President of the United States is in desperate need of surgery -- and the operation must be kept secret. Naturally, Seaview is chosen as the best place for the operation. An enemy agent, taking the place of the doctor who would have used a radiological device on the President, will attempt to kill the Commander-in-Chief with a deadly ray. Will she be discovered in time to save the President?

Nelson's sister is abducted by enemy agents who demand top-secret information as ransom, but will Nelson betray his country to protect the only family he has?

Crane is caught up with a circle of spies as he tries to make his way back to Seaview with important missile defense data.

finds herself suddenly out of touch with the rest of the world, in the wrong part of the ocean and encountering strange creatures. It seems she's been thrown a million years back in time. Then the Admiral opens his cabin door and finds himself back in his Santa Barbara office, confronting the strange Mr. Pem and his pocket-sized time machine, and has to play a risky game before time can return to its normal course. The early part of this episode contains two astronomical errors. First, Nelson takes snapshots of the eclipse-darkened sky with an ordinary camera, but when the shots are developed they show galaxies and nebulae that could only be seen with a telescope. Second, the Admiral describes the pictures as showing 'constellations'. Henry Jones (Pem) also appears as Sprague in "Night of Terror" and once again as Pem in "No Way Back". This episode features re-used footage from "Thing from Inner Space".

Seaview experiences a sudden power overload and blows up, killing all hands aboard. When Admiral Nelson (who was at the Nelson Institute at the time of the accident) hears of the destruction of the Seaview, he gets an impossible to refuse offer from Mr. Pem (who survived from A Time To Die): in exchange for the use of the sub's nuclear reactor, Pem will transport Nelson back in time to before the explosion which destroyed Seaview. Once back in time, Pem renergizes his time travel device and brings the sub back even further in time to the American Civil War. Before the crew has time to absorb this, the ship is boarded by Major General Benedict Arnold! Now Nelson has to stop Pem's far-reaching plan to take over the world and avoid the explosive disentergration of Seaview.

A Soviet satellite has crashed into the sea and its nuclear reactor must be disarmed before it causes a disaster. Two Soviet scientists come aboard Seaview to carry out the work -- but one of them is an imposter.

After Nelson's experimental submarine is attacked by a giant jellyfish, tension mounts aboard the Seaview as Nelson exhibits signs of a mental breakdown.

Nelson and Crane hear about the destruction of the submarine Angler. When they read the captain's log and hear the tapes made in the control room, Nelson and Crane are puzzled: Captain Williams wen insane and machine-gunned officer Richardson, yet both men were close friends. It seems that after they had left a mysterious island, the men became deadly enemies.

Nelson is injected with a radioactive material that makes him a walking time bomb. This episode marks the first appearance of the FS-1!

A space probe returns to earth and is recovered by Seaview...only to unleash an alien organism aboard.