Zale Parry

Before Mike Nelson can investigate the disappearance of two geologists the sea washes one of them upon the beach, his body bearing marks apparently made by the suction valves of a giant octopus. When later the body of a young boy is found with the same mysterious marks, Mike is sure that the killer is a diabolically clever man who has somehow found a way to duplicate the mark of an octopus.

Lewis Hale tells Mike that he has invented a miraculous machine which locates sunken ships laden with gold. Mike discovers the machine is actually a stolen Top Secret U.S. Navy device, and pitches into a life-or-death struggle with Hale to retrieve it.

Six sportsmen rent an amphibian plane to fly them to a good skin-diving site. Mike Nelson is hired to supervise their safety. When one of the divers, using a Sea Scooter, mysteriously disappears, Mike searches and finds him - photographing secret Navy installations! Mike Nelson, armed only with a dagger, competes for his life against the spy's powerful spear-gun.

Skin-diving fans Robert and Maria Tyler explore the sea with an underwater camera. But only Mr. Tyler returns. His explanation: wife Maria drowned "accidently." Mike Nelson uses an underwater photograph to test the truth of Tyler's story.

A local fisherman wages war against kelp harvesters, believing they're destroying his fishing grounds. But when his protests escalate to violence and a planned bombing, Mike must risk his life to prevent the attack.