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.
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.
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.
Mark encounters Muldoon, who is using a sea monster to make island natives sacrifice young girls to him.
C.W. develops a Jekyll and Hyde personality after swallowing a mysterious liquid.
An underwater landslide transports Mark to Verona, Italy ... in the days of Romeo and Juliet.
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.
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.
Using his genetic scientists, Schubert produces a giant jellyfish which he intends to release unless his extortion demands are met.
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.
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.
When a multi-million 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.
Schubert threatens to cause world wide flooding by using powerful microwaves to melt the earth's polar ice caps unless the government turns Mark Harris over to him.
