Nelson works on finding an assassin before he can kill the new Secretary of State.

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.

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

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.

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?

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.

The Seaview is being buffeted about by underwater explosions due to the eruption of an active volcano. Dr. Turner (Victor Jory) is aboard to stop the eruptions. However, his true mission is to bring up ""elixir stones."" An alchemist, Turner needs the stones to keep him immortal. However, in order to bring up the stones, he needs to stimulate the volcano into full eruption!

"There's nothing to do here but to keep from disappearing." Lt. Commander Morton Seaview is carrying a new missile, and Nelson has orders to deliver it to its launch site at all costs. Unfortunately, the missile malfunctions, and Nelson can't get into the sealed silo to fix it. Soon, the crew starts to disappear, and bizarre phenomena plague the ones who remain.

Doctor Blanchard has created a device to control the actions of a white gorilla. Unfortunately, his assistant Leo Brock wants the fame that the inventor of such a device would gain. As a result he uses the device to enrage the gorilla and kill Blanchard. Nelson gets there too late to save Blanchard, but the gorilla is taken on board. Brock sets the gorilla free and he runs amock until both Brock and the gorilla are killed.

A man from the future sends two androids back in time to kidnap Nelson from Seaview in order to add him to a collection of other famous military figures abducted from various eras of the past. The plot of this episode is virtually identical to the plot of the ""Time Tunnel"" episode entitled ""The Kidnappers"".

A scientist experimenting with plant life on a remote island has run into trouble. A shore party from Seaview arrives only in time to hear his dying warning that the world will be taken over in eight hours. Nelson brings aboard the only unusual thing he can find -- a red orchid, which proves to be a malign alien (that proceeds to take over his mind and those of the crew).

Seaview has been sent to the ocean floor by mysterious pressure impacts. Crane, Sharkey, and Kowalski go out in the Flying Sub to investigate. They, too, are sent to the bottom of the sea. Strange creatures (half men-half fish) present themselves and explain that they intend to take over Seaview (who doesn't?) and eventually the world! Don Matheson starred as Mark Wilson in Land of the Giants.

About the plot . . . Um . . . Well . . . Uh . . . Okay, here it is: You see, there are these twin leprechauns, one good, the other icky. The icky one wants all the gold that's buried beneath an underwater defense installation in the Irish Sea. The good leprechaun does his best to stop his evil brother from doing any harm . * Footage of Morton and Kowalski trapped in an underwater cave was borrowed from previous episodes. So what else is new? * Why this episode wasn't broadcast on St.

"You had no right -- no right to make that second dive without orders from me!" Nelson to Crane Another scientist with another artificial atmosphere (see Season 1's "The Condemned") that will revolutionize deep-sea diving. When will Admiral Nelson learn! Naturally, Captain Crane is used as a guinea pig for these dangerous experiments. And there are fatal side-effects. * A great Nelson/Crane argument takes place in the Admiral's cabin after the Captain has made his unauthorized second dive. * In this episode it is not Nelson who shoots Crane, but rather, Chip Morton. * Notice the tears of gratitude and relief in Crane's eyes after Nelson gives him the antidote at the very end of the episode. * Crane also shed a tear (from pain) in Season 3's "Day of Evil".

Aliens are at it again. They're planning to destroy the Earth. However, one of the aliens claims to be a good guy. Can he be believed? Will mankind survive yet another close encounter with space visitors? * Skip Homeier also appears in "The Amphibians" and "The Day the World Ended" * Kevin Hagen guest-starred in "The Shape of Doom".