The Seaview is sent to stop a warhead on the ocean floor from detonating. But a leprechaun wants it to go off. So he tries to stop them. But his good twin brother tries to help them stop him.
A lobster man arrives in a space craft claiming to be a friend but in reality he wants one of the nuclear rods to cause an explosion thus paving the way for an invasion.
Crane receives a Mayday from the Seaview and finds no one on board except a UFO expert. The crew re-appears and announces they are going to fire nuclear missiles at Washington D.C.
The Seaview rescues two scientist from a climate control project at the South Pole but something else is on the ship on a killing spree. The Seaview has to shut down the project before the world's climates change.
The Seaview and an intelligence expert who is more than he seems, try to find a group that is blackmailing the world's nations for gold bullion. The submarine encounters gigantic creatures during their search.
The Seaview investigates why part of the Italian coast has become overrun with jungle growth. Meanwhile new crewman Keeler has 3 miniature silver faced men grow to human size and causes jungle growth to spawn all through the submarine.
Investigating why the world's oceans are rising, the Seaview finds a race of Frost Men at the Polar Cap who are increasing the temperature and they want the submarines' reactor to help them leave the earth.
While investigating the destruction of the fleet by a flying saucer, Nelson and Kowalski are captured by aliens. One of the aliens offers help to the Seaview crew to stop his people.
Corpsman Mallory's 4th Dimension Machine causes Captain Crane to kill Admiral Nelson, but did he really? A hunted Crane tries to prevent the event from happening.
Mr.Pem returns to help Admiral Nelson prevent the Seaview from exploding. Pem transports the submarine to 1780 where they encounter General Benedict Arnold and Major Andre.