A scientist and his secretary come aboard Seaview when their research vessel is sunk by a giant Portugese Man-Of-War. They want Admiral Nelson to return to the scene, but won't tell him the real reason.

Captain Gantt, an ego-maniacal scientist, is in a nuclear sub testing a powerful new fuel. Due to his bungling, the sub crashes into a giant web and is attacked by a monsterous sea spider. The Seaview investigates and rushes to retrieve the remaining fuel canisters before the enormous underwater pressure collapses the test sub abd detonates the fuel - an explosion which would kill millions!

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.

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.

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.

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?

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.

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?

Seaview is on the lookout for an enemy sub and its underwater sub-pen. Sonar picks up a contact. The rogue sub fires at Seaview, which is saved only by virtue of her new electronic hull shield. Not so fortunate is Captain Crane, who has just located the sub-pen while on patrol in the FS-1. A powerful laser fires a beam at the Flying Sub, crippling the ship. Running out of air, moving in and out of consciousness, Crane is unable to give the pen's coordinates to Nelson. Thanks to the hard work of some enemy agents aboard, Seaview seems unable to rescue the Captain. Worried about Crane trapped in the Flying Sub, Admiral Nelson is particularly impatient in this episode. He constantly barks at Morton and the Chief.

The Seaview is en route to rendezvous with the fleet when Sharkey reports a missle fuel leak. Crane and Morton believe it's worth returing to port to repair, however Nelson knows how to handle it. Taking a small breathing device with him, the Admiral locks himself in the compartment in question to effect repairs. However, he drops a wrench, the circuts blow and Nelson is knocked out. When he comes to, he's gripped by paranoia which worsens with each passing minute. Trusting no one, Nelson loses his grip on reality, arresting Doc, fighting with Sharkey and finally launching a nuclear missle at the fleet!

The ghost of Blackbeard is alive and well, and he wants the Seaview and her crew. The pirate 'drafts' Kowalski, who makes a very convincing and quite blood-thirsty buccaneer. Nelson and Crane have their hands full trying to protect the President of the United States (a guest of a Middle Eastern Shah on a nearby yacht), and regain control of their ship.

A lobster-like alien emerges from a capsule retrieved from the ocean floor. He claims that all he wants is one of Seaview's nuclear rods to power his ship for the homeward journey, but neither Captain Crane nor Admiral Nelson is convinced he can be trusted. Victor Lundin also played the neck-braced Hansjurg in Season 2's "The Menfish".

"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.