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.

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.

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 that Billy once had the same webbing as Mark, but also that he has had it removed.

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

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.