While investigating the disappearance of research ships in the Aegean, the Seaview is damaged by a bomb. Captain Crane goes ashore, and pretends to be a diver for hire. The Captain meets an ex-diver, now a mute due to a tragic accident, who claims to have seen a ""city beneath the sea"". Crane gets hired by the ""bad guy"", but his cover is soon blown. He is taken prisoner and nearly loses his life.

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.

On a rendezvous mission, Crane and several crewmen are drugged into unconsciousness on a deserted yacht. Waking up in a South American prison, the men are taken out to the courtyard, one per hour, and executed. All will die unless Crane breaks his oath and confesses what he cannot. Will Nelson's rescue mission succeed? Will the country in question survive the machinations of the evil man who wants to take over?

A husband and wife Antarctic research team is killed by an experiment on plankton gone wild. Seaview arrives to investigate and take plankton samples. Soon the submarine is in danger of succumbing to the horrible, slimy growth. Nelson, Crane and crew face another problem: one of the three scientists on board the sub is an enemy agent.

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.

A lush, tropical, verdant paradise in the South Pole? Living dinosaurs? Such fanciful ideas must surely be the ravings of a madman. Or perhaps not, as Admiral Nelson soon discovers. The question is, will he and Crane survive their trip to a lost world?

A sea monster kills three villagers in a Norwegian fjord; however, one man survives. Seaview arrives to investigate. Nelson and Crane are met with hostility from the locals and find the survivor has died. Nelson does his best to make sure the ""sea monster"" doesn't destroy 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.

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?

A mysterious weapon is destroying American U2s. The Seaview's mission is to find the weapon and destroy it. Captain Crane goes undercover. A night-club performer, who is also a resistance fighter, comes to Crane's aid. The Captain is soon captured and severely beaten. Meanwhile, on Seaview, Major Amadi demands asylum. To prove his sincere intentions, he promises to lead the sub to the powerful magnetic weapon responsible for taking out those U2s. Nelson suspects he wants to lead Seaview to a watery grave.

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.

In blizzard-wracked Florida, Nelson contacts a meteorologist who may be able to help explain the strange weather conditions. That night, however, the scientist is abducted, and ""programmed"" to kill Nelson. When his first attempt fails, he comes aboard Seaview when she sails to investigate the source of the problem, and continues his efforts.

An old friend of Nelson's comes aboard Seaview to continue his research into the giant whale that destroyed his ship, crippling him and killing his son. His real motives, however, have more to do with vengeance than science, and Crane becomes concerned that the Admiral is caught up in his friend's obsession.

When the King of a small but friendly nation is assassinated, Seaview is assigned to take the young Crown Prince back home to assume his throne. Neither Nelson (who gets turned out of his own cabin) nor Crane are used to handling children. Help arrives in the form of a mysterious flute-playing old man who befriends the boy after he is picked up at sea.

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?

Admiral Nelson is kidnapped from a plane during mid-flight and held on a remote island by Nazis who plan to conquer the world.

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

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?"

An undersea earthquake exhumes hundreds of strange metallic capsules, and a powerful humanoid creature who could destroy mankind is restored to life after millions of years in suspended animation.

After returning from a mission on deep space, a lifelike robot is brought aboard the Seaview, but the crew soon discovers that the robot has been mysteriously reprogrammed to wreak havoc.

A mad art connoisseur seizes control of the Seaview and plans to pirate Leonardo da Vinci's Mona Lisa from a French Ship bound for the World's Fair.

A newly installed super-computer is tested on the Seaview...but the enemy has other plans.

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

A strange experiment to speed up evolution goes horribly awry, and an enormous protoplasm monster threatens the lives of the entire crew.

The cre of the Seaview is called to investigate an undersea laboratory where scientists are surgically transforming humans into amphibians.

Nelson is stranded aboard a life raft with the ex-premier of a hostile foreign power who carries vital information on a plot to destroy the free world.

The crew is joined by Captain Adams, the sole survivor of an ill-fated missile launch, in search of the mysterious force responsible for the disaster.

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.

The Seaview travels to Scotland via a secret, underground conduit to investigate the mystery of the legendary Loch Ness Monster.

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.

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?