After a violent storm at sea, the inert body of a man is found on the beach near the Naval Undersea Center. Equipped with webbed hands and gills instead of lungs, he can breathe underwater, swim faster than a dolphin and dive to depths of at least seven miles. He is nursed back to health by Doctor Elizabeth Merrill and given the name of Mark Harris. In return, Mark agrees to help the United States Navy recover a lost missing submarine carrying top military officials.

Mark investigates the disappearance of three scuba divers, two of whom are 'replaced' by waterbourne aliens (Tiffany Bolling as Lioa / Dilly and Burr DeBenning as Xos / Chazz). The aliens assignment is to scout Earth and check its defense capabilities. They also give the impression that Mark may also be one of them, specially bred for Earth infiltration, but this is not specifically confirmed.

When a space capsule crashes in the ocean near the Cetacean, Mark and Elizabeth agree to investigate. When they arrive, Mark is rendered unconscious by a screeching sound. They discover hundreds of strange, blue, intelligent spores that have taken over Mark's body, attempting to find a way back into space. However, they do not know that Mark will die if he does not return to water within a few hours.

Elizabeth is kidnapped, one of dozens of scientists held prisoner on the island of Felicitos, controlled by special mineral springs that render their victims completely happy and compliant. Dr. Smith is using them to build a rocket to take her away from our troubled planet in search of some better world, and Mark must find a way to counter the brain-washing and free the captives.

Schubert threatens to cause worldwide flooding by using powerful microwaves to melt Earth's polar ice caps, unless the government turns Mark Harris over to him.

When a multimillion-dollar underwater probe malfunctions and begins attacking any ship that travels near it, Mark must reason with its highly advanced brain and convince it to stop.

Mark, investigating a power outage, discovers a centuries-old hawk statue from the legendary civilization of Mu. The statue, when used properly, can remove the power from a large area. Schubert discovers the power of the hawk statue and Mark must prevent him from taking it.

The oceans are leaking through a fissure in the ocean floor. A con-man named Muldoon agrees to guide Mark through the fissure to investigate. However, Mark is unaware either that giants inhabit the other side of the fissure or that Muldoon has already stolen gold from one of them.

Schubert threatens to cause worldwide flooding by using powerful microwaves to melt the earth's polar ice caps, unless the government turns Mark Harris over to him.

When a multimillion-dollar underwater probe malfunctions and begins attacking any ship that travels near it, Mark must reason with its highly advanced brain and convince it to stop

Mark, investigating a power outage, discovers a centuries-old hawk statue from the legendary civilization of Mu. The statue, when used properly, can remove the power from a large area. Schubert discovers the power of the hawk statue and Mark must prevent him from taking it.

The oceans are leaking through a fissure in the ocean floor. A conman named Muldoon agrees to guide Mark through the fissure to investigate. However, Mark is unaware that giants inhabit the other side of the fissure and Muldoon has already stolen gold from one of them.

Using his genetic scientists, Schubert produces a giant jellyfish which he intends to release unless his extortion demands are met.

Mark is somehow linked with a man named Billy, existing in a wild west town, who appears to be his twin. Investigating, Mark arrives in the town and discovers Billy had the same webbing as Mark but has had it removed.

Schubert attempts to make a satellite weapon to knock out the Earth's communications. However, to power the weapon he needs the energy crystals protected by a forcefield under the ocean. The crystals actually power the forcefield that protects an underwater city.

An underwater landslide transports Mark to Verona, Italy... in the days of Romeo and Juliet.

C.W. develops a Jekyll-and-Hyde personality after swallowing a mysterious liquid.