Seaview is investigating a thousand-mile wide communication blackout. The source of this is a large alien brain found under the sea. It is broght aboard and takes over visiting scientist, Dr. Lindsay. With his help, and after possessing Captain Crane, the alien vows to take over the Seaview and after that, the Earth.

Two scientists examining a radioactive volcano are attacked by a wolf. One dies, and the survivor is infected by a virus that causes him to change from man to werewolf and back again. Admiral Nelson is exposed to the virus and the crew must find a vaccine.

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.

Captain Crane searches for a lost diving bell containing Admiral Nelson, Chief Sharkey, and Dr. Sprague.

Seaview is investigating a UFO. Along the way, they pick up a shipwrecked old man -- almost always a bad omen when our gallant crew finds a castaway! The bedraggled fellow has a sack full of strange, frightening toys capable of murder, and of destroying the submarine. Turns out the alien ship needs to melt down Seaview's hull for fuel. Nelson and Crane do their best to foil the aliens' plans before it is too late.

A UFO crashes and an alien assumes the form of Admiral Nelson.

Captain Crane rescues a diver from a wrecked atomic sub. He turns out to be Kowalski's brother, Stan, a Navy diving specialist. When the Seaview needs his help in getting off the ocean floor, Stan is not up to the job.

Bainbridge Wells is filming a segment of his "Science On The Move" television series on a small island off the coast of Chile. A sea creature comes ashore and kills his crew. Wells convinces Admiral Nelson to return to the island because he wants to prove that the creature really exists. Crewman Patterson has a special interest in the mission.

Admiral Nelson and Captain Crane are involved in a mind-control experiment, with Chief Sharkey caught in the middle.

Faceless aliens invade Earth and try to take over an underwater atomic base. The leader takes the form of an old, deseased friend of Admiral Nelson in order to convice Nelson they mean no harm. Nelson will have none of it and leads a landing party to the base to stop the aliens, even if the cost includes their own lives.

A ghostly ancestor of Admiral Nelson visits him aboard Seaview.

Twin scientists - one good, one evil - come aboard Seaview. The evil twin is out to create a radioactive "plant man."

While racing to help a foundering research vessel, the Seaview's nuclear reactor goes critical. Admiral Nelson's right hand is exposed to intense radiation as he fixes the problem, causing a flare-up of the "werewolf" virus that he carries.

The Seaview is called in to help track an artificially created life form, now loose in the Pacific. The creature, which is growing at a fantastic rate, has its own agenda -- starting with a takeover of the crew.

As World War II draws to a close, a secret underwater Nazi base is damaged during a sea battle. A Nazi scientist and his aide are accidently put in suspended animation. Thirty-five years later, the crew of the Seaview discovers and revives them. The Nazis attempt to carry out their interrupted mission: destroy the capital cities of the Allies.

Arctic Listening Station XZ-5 receives a message from a voice from space (""laser beeeeeam""). Professor Bergstrom, ignoring the advice of his assistant, activates their beam and brings the alien, living flame, to Earth. Now, this Heat Monster wants the Seaview to detonate a bomb under the polar ice cap. How can the intrepid crew defeat this new menace?

The crew of a lost submarine returns as rock men to menace the Seaview. A new man, Kowalski's friend Richards, is captured on a diving mission and turned into a Fossil Man. Nelson is then ordered to turn command of the Seaview over to these rock men. Will Nelson comply? Will the crew be dipped into the transformation lava to exist as living rock?

As the Seaview conducts a marine census, Captain Crane spots a mermaid. He brings her aboard and falls under her spell. A sea creature follows the mermaid and proceeds to cause big trouble on the sub. The problems are compounded when the Seaview is ordered to disarm a nuclear device that has been planted off the coast of California.

The Seaview is on a top secret mission to transport a 3,000 year old sarcophagus from New York to a country in the Middle East. The United States hopes that by returning a national treasure, war will be prevented. The inhabitant of the sarcophagus has other ideas, and attempts to prevent the Seaview from reaching her destinaton.

The Seaview takes on a skeleton crew and is being used to guide a new interstellar space probe. However, Chip Morton is taken over by an alien shadow man which wants to stop the probe.

While searching the deep for an enemy complex, the Seaview is rammed by another sub. The crew rushes to make repairs before the air runs out and a lurking monster swallows a diving party led by Crane.

While Nelson and Sharkey search by air for an uncharted island, Seaview brings aboard a gigantic egg from the bottom of the sea during a routine specimen collection. It later hatches a fully grown, orange creature which kills a background guy and tosses Kowalski around the missle room. Nelson returns and (in a very un-urgent manner) has the crew hunt down this grunting thing as it terrorizes the crew. Later, Crane and Kowalski take the flying sub out to investigate the area, crash landing near the uncharted island. Another orange crature (one that speaks and is the leader) captures them and brings them to creature HQ. The monsters are aliens from Scorpious which are trying to hatch millions of eggs. Can the crew stop this invasion before these amphibian aliens take of the Earth?

The Seaview is carrying Atlantanean statues in its missile room as a favor for the State Department. As they wait for Captain Crane to return, a midget clown emerges from one of the crates and begins sending forth his animated waxwork duplicates of the crew. Crane's only hope is to try to revive Admiral Nelson and retake the ship. But, can they?

A mysterious cloud is enveloping the Earth and causing massive Tsunami and other catastrophic environmental damage. Crane takes the flying sub in to find out the cause and is taken over by an evil alien. Once aboard, this alien tries to stop Nelson before he can destroy the cloud with nuclear weapons.

Admiral Nelson has been brainwashed. After getting a vital energy source, he attempts to kill Crane and the visiting Dr. Land in an undersea cave. He then returns to the sub, gasses the crew and sets the reactor to blow. Can Crane recover and return to the Seaview before Nelson can destroy it?