After the war is over, a radioactive cloud begins to sweep southwards on the winds, gradually poisoning everything in its path. An American submarine captain is among the survivors left sheltering in Australia, preparing with the locals for the inevitable. Despite his memories of his wife, he becomes close to a young woman struggling to accept the harsh realities of their situation. Then a faint Morse code signal is picked up, transmitting from the United States and the submarine must set sail through the bleak ocean to search for signs of life.