The story is set primarily in and around Melbourne, Australia, in 1963. World War III has devastated most of the populated world, polluting the atmosphere with nuclear fallout, and killing all human and animal life in the Northern Hemisphere. The war began with a nuclear attack by Albania on Italy, and then escalated with the bombing of the United States and the United Kingdom by Egypt. Because the aircraft used in these attacks were obtained from the Soviet Union, the Soviets were mistakenly blamed, triggering a retaliatory strike on the Soviet Union by NATO.There is also an attack by the Soviets on the People's Republic of China, which may have been a response to a Chinese attack aimed at occupying Soviet industrial areas near the Chinese border. Most, if not all, of the bombs included cobalt to enhance their radioactive properties.Global air currents are slowly carrying the lethal nuclear fallout across the Intertropical Convergence Zone, to the Southern Hemisphere. The only parts of the planet still habitable are Australia, New Zealand, South Africa, and the southern parts of South America, although they are slowly succumbing to radiation poisoning as well. Life in Melbourne continues reasonably normally, although the near-complete lack of motor fuels makes traveling difficult.People in Australia detect a mysterious and incomprehensible Morse code radio signal originating from the American city of Seattle, Washington. With hope that someone has survived in the contaminated regions, one of the last American nuclear submarines, USS Scorpion, placed by its captain, Commander Dwight Towers, under Australian naval command, is ordered to sail north from its port of refuge in Melbourne (Australia's southernmost major mainland city) to contact whomever is sending the signal. In preparation for this journey, the submarine makes a shorter trip to port cities in northern Australia, including Cairns, Queensland and Darwin, Northern Territory; no survivors are found. Two Australians sail with the American crew: Lieutenant Commander Peter Holmes, naval liaison officer to the Americans, and a scientist, Professor John Osborne.