Nelson and Crane go undercover and risk their lives in order to bring a Communist defector back to the United States. But the man is injured in the attempt, and his lover is left behind. He won't speak to the agent waiting for him on Seaview (actually a double agent) until he is reunited with his Anna. But even Anna is not all she seems.

Admiral Nelson is kidnapped from a plane during mid-flight and held on a remote island by Nazis who plan to conquer the world.

After returning from a mission on deep space, a lifelike robot is brought aboard the Seaview, but the crew soon discovers that the robot has been mysteriously reprogrammed to wreak havoc.

Crane is brainwashed by enemy agents, and his new mission is to deactivate certain strategic undersea missiles and to assassinate Admiral Nelson!

The cre of the Seaview is called to investigate an undersea laboratory where scientists are surgically transforming humans into amphibians.

Nelson and Crane hear about the destruction of the submarine Angler. When they read the captain's log and hear the tapes made in the control room, Nelson and Crane are puzzled: Captain Williams wen insane and machine-gunned officer Richardson, yet both men were close friends. It seems that after they had left a mysterious island, the men became deadly enemies.