After a storm, a young man is found washed up on a beach. Nearly dead after hours of of the life sustaining ocean water, he is nursed back to health by Dr. Elizabeth Merrill and eventually agrees to undertake a hazardous mission to locate a missing sub. Several miles beneath the sea, he finds an underwater habitat, where he confronts loopy scientist Mr. Schubert for the first time.
Now working for the Foundation for Oceanic Research, Dr. Merrill and Mark try to discover clues to his origins. Called in by the coastguard to investigate the disappearance of three scuba divers, they encounter two water-breathing humanoids who Mark thinks could be his people, but who turn out to be scouts from an alien civilization intent on colonizing Earth.
A NASA space probe returns to Earth contaminated with ectoplasmic matter that is capable of invading humans, placing them in a catatonic stupor or forcing them to do things against their will. The spores, visible only to Harris because of the unique structure of his eyes, communicate with him and demand that he help them return to space or see mankind destroyed.
When Dr. Merrill is kidnapped, Mark Harris and Miller Simon must locate a remote island off the coast of South America and penetrate the carefully guarded complex of mad scientist, Dr. Mary Smith.
Villainous Mr. Schubert causes the world's sea level to rise alarmingly, by melting the polar ice caps, and won't stop unless Mark submits himself to his genetic study.
Mr. Schubert loses control of his latest invention - a robot-like machine designed to gather a radioactive mineral from the sea bed. Discovering that the 'Mudworm' has taken on 'human' characteristics, Mark tries to reason with it.
After Mark rescues an ancient stone hawk from its watery crypt, evil Mr. Schubert steals it for his own malevolent purposes, unaware that the statue has the power to come to life and cause worldwide electrical blackouts.
Investigating a 'leak' in the ocean, Mark is swept into the inner world where he meets Thark, a nine-foot-tall gold prospector whose sluice mining is draining the world of its water. Realizing the threat, he and Mark join forces to close off the sluice gate. Complicating matters is a gold-seeking conman called Muldoon.
It's the Man from Atlantis vs. the world's biggest jellyfish when Mr. Schubert plots to extort money by sabotaging an international swimming gala.
A time warp in the ocean throws Mark back to the old West, where he encounters his twin – desperado Billy Jones – who is identical in every way, except that he has scars where the webbing on his hands has been removed.
Investigating a disturbance in the Pacific, Mark finds Mr. Schubert trying to get his hands on some precious crystals located within a force field. Penetrating it, Mark encounters a mysterious white-skinned, white-suited race called the Deepspeople, who talk in a strange, clicking language.
Mark travels back in time to 14th-century Italy, where he gets involved with Romeo and Juliet, 'rewriting' the story to give it a happy ending!
C. W. Crawford accidentally swallows a deep-sea enzyme which alters his appearance and mind, resulting in Jekyll and Hyde personalities.
Investigating a loss of five poisonous gas canisters, Mark Harris re-encounters conman, Jake Muldoon, and his tame, two-legged, two-headed giant sea monster, called Oscar.
Mark and his friends tangle with Moby, a water-breathing imp whose magic touch makes his victims revert to their childhoods – with sometimes fatal results.
Mark tangles with a mermaid who can 'capture' humans with her nerve-penetrating wail. She has been kidnapped by a modern-day pirate, Stringer, who is using her to lure mariners to his lair.
Mark infiltrates an ocean-side carnival – where a government agent was working before he was found mysteriously drowned – and foils a plot to steal a priceless Egyptian artifact from a museum.