After Seaview's sister ship, Polidor, is lost on an experimental deep dive, Seaview continues with the tests. Unknown to the already stressed crew, one of the visiting psychologists aboard is an enemy agent who has released a fear-causing gas into the air system. As Seaview plunges into the depths a dangerous panic starts to spread.

On a rendezvous mission, Crane and several crewmen are drugged into unconsciousness on a deserted yacht. Waking up in a South American prison, the men are taken out to the courtyard, one per hour, and executed. All will die unless Crane breaks his oath and confesses what he cannot. Will Nelson's rescue mission succeed? Will the country in question survive the machinations of the evil man who wants to take over?

An alien spaceship lands in the ocean, and Seaview is sent to investigate. Although Seaview is powerless in the vicinity of their ship, the aliens seem friendly, and Nelson manages to open negotiations. The military authorities, however, have other ideas, and Seaview may be in as much danger from human interference as from the aliens.

Damaged by an explosion from an old mine field, Seaview takes a dive to the ocean floor. Listing dangerously, running out of oxygen, having to deal with fire and flood, the crew can do nothing but wait for help. But will help arrive in time?

A mysterious weapon is destroying American U2s. The Seaview's mission is to find the weapon and destroy it. Captain Crane goes undercover. A night-club performer, who is also a resistance fighter, comes to Crane's aid. The Captain is soon captured and severely beaten. Meanwhile, on Seaview, Major Amadi demands asylum. To prove his sincere intentions, he promises to lead the sub to the powerful magnetic weapon responsible for taking out those U2s. Nelson suspects he wants to lead Seaview to a watery grave.

Nelson and Crane are dismayed when an egotistical, publicity-seeking scientist is given command of the Seaview to test a new pressure system which will allow the sub to explore uncharted depths.

A toy manufacturer is involved with kidnapping and shipping politicians and scientists to foreign powers.

A submerged domed island is forced to the surface by its volcano and exposes a Jurassic-style world.

Admiral Nelson and Captain Crane are involved in a mind-control experiment, with Chief Sharkey caught in the middle.