After a violent storm at sea, the inert body of a man is found on the beach near the Naval Undersea Center. But this is no ordinary man. Equipped with webbed hands and gills instead of lungs, he can breathe underwater, swim faster than a dolphin and dive to incredible depths. He is nursed back to health by Doctor Elizabeth Merrill and given the name Mark Harris. In return Mark agrees to work for the Foundation of Ocean Research and his first assignment is to seek out a missing submarine carrying top military officials. Swimming to incredible depths, Mark discovers an enormous undersea habitat constructed by Mr. Schubert, a maniacal scientist who plans to destroy all the nations of the world with their own nuclear weapons through an ingenious method of mind control.

Mark investigates the disappearance of three scuba divers and becomes involved with a pair of aliens who are the advance force of an alien invasion.

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 take over Mark's body attempting to find a way back into space. However they don't know Mark will die if he does not return to water within a few hours.

A demented scientist, Dr. Mary Smith, abducts and drugs top scientists from around the world -- including Elizabeth Merrill -- to work on a secret project.

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

A demented scientist, Dr. Mary Smith, abducts and drugs top scientists from around the world — including Elizabeth Merrill — to work on a secret project.

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.

Mark encounters Muldoon, who is using a sea monster to make island natives sacrifice young girls to him.

An impish little man enters an underwater station and causes the crew to become irrational simply by touching them. After three crewmen die the F.F.O.R. hears Duke, the last crewman, talking over the radio in a childish manner and investigates. Mark swims over and brings Duke over to the Cetacean, not knowing the imp has stowed aboard.

While investigating the mysterious loss of three ships in one part of the ocean, Mark and the crew of the Cetacean encounter a submarine operated by a modern-day pirate. The pirate has captured a mermaid that can produce a hypnotic siren song, which mesmerizes anyone who hears it, even Mark.

Mark goes undercover to investigate members of a carnival planning to break into a museum. The only way to break into the museum is through an underwater tunnel of which only Mark can swim. When he is approached, he refuses. The owner of the carnival is then kidnapped and threatened unless Mark helps.