As the U. S. O. S. Seaview , a mammoth glass-nosed atomic submarine designed by scientist Harriman Nelson, makes its trial run near the polar icecap, the Van Allen radiation belt suddenly bursts into flame and threatens to destroy the Earth. Nelson is convinced that the only hope for survival is to shoot a Polaris missile into the belt and thereby cause it to explode backward into space. When the United Nations rejects his proposal as being too dangerous, Nelson commandeers the Seaview and heads for the Marianas, where he plans to launch the missile. Also aboard the submarine are Susan Hiller, a psychiatrist studying the effects of prolonged confinement on human behavior; Cathy Connors, Nelson's devoted secretary; Lucius Emery, a noted physicist who concurs with Nelson's theory; Capt. Lee Crane, the Seaview 's skipper; Miguel Alvarez, a civilian scientist; and Chip Romano, a brash young officer. Once the vessel is underway, several of the personnel begin to question Nelson's sanity; and there are repeated attempts at sabotage. After a run-in with a giant squid and a passage through a World War II mine field, the Seaview is attacked by U. N. submarines sent to prevent the launching of the missile. But Nelson, knowing that the Seaview can withstand depths far greater than any other undersea vessel, takes his craft down deeper and deeper until the tremendous pressure causes the pursuing submarines to explode. When the Seaview reaches its destination, Susan is revealed to be the saboteur. Convinced Nelson's actions will destroy the world, she makes a last effort to prevent the launching; but she is accidentally killed. Crane then sets the detonator, and the Polaris is sent into space. The wild experiment proves successful as a shattering explosion restores the burning sky to a tropical stillness.