The new, state of the art nuclear submarine ''USOS Seaview'' is on diving trials in the Arctic Ocean. The ''Seaview'' is designed and built by scientist and engineering genius Admiral (United States) Harriman Nelson (USN-Ret) (Walter Pidgeon). Captain (naval) Lee Crane (Robert Sterling) is the ''Seaview's'' Commanding Officer. One of the on-board observers is Dr. Susan Hiller (Joan Fontaine), studying crew-related stress. The mission includes being out of radio contact for 96 hours while under the Arctic ice cap, but the ice begins to crack and melt, with boulder-size pieces crashing into the ocean around the submarine. Surfacing, they discover fire burning in the sky. After the rescue of scientist Miguel Alvarez (Michael Ansara) and his dog at Ice Floe Delta, the sub receives radio contact from Mission Director Inspector Bergan at the Bureau of Marine Exploration. He advises that a meteor shower pierced the Van Allen radiation belt causing it to catch fire, resulting in a world-threatening increase in global warming. Nelson's on-board friend and scientist, retired Commodore (United States) Lucius Emery (Peter Lorre) concurs that it is possible. Bergan informs Nelson that the President wants him at a UN Emergency Scientific Meeting as soon as possible.