Peter Holmes, an Australian naval officer, wakes up happy because today he will meet with the Navy Department in Melbourne, the southernmost large city in the world. He expects the meeting to lead to a new appointment and his first work in seven months. He has not worked since the "short, bewildering war" ended.

The nuclear war started in the Northern Hemisphere and lasted for thirty-nine days. Peter was on a Navy ship during the war, but he is now back home in Falmouth, a suburb of Melbourne, with his wife, Mary, and his baby daughter, Jennifer. Since the war ended, there is no more gasoline for cars, so Peter rides his bicycle to a farm to get milk. He then takes his bicycle over to the train station, where the parking lot is full of horses instead of cars.

Peter takes the train to Melbourne and goes to the offices of the Navy Department. He meets with an admiral who assigns him to the position of liaison officer aboard the U.S.S. Scorpion, an American nuclear submarine. Peter has already met Dwight Towers, the submarine captain, who is a pleasant, affable man.

When the war started, Dwight received orders from the U.S. Navy to take the Scorpion to Manila. Four days later, his instruments detected radioactive dust above the empty sea. He was unable to establish contact with any American radio station or ship. When Dwight established contact with Australia, he took the sub south. On his way down, he came across an American cruiser. For the first time, he learned about the war and its origin—newspapers and radio stations had ceased operations during the war, so many people were left without knowledge of how or why everything started. Dwight learned that Albania initiated an Arab-Israeli war, which led to a Russia-NATO war, which led to a Russo-Chinese war. Atomic bombs fell all over the Northern Hemisphere.

Peter learns he will be posted to the Scorpion for a year. During that time, the submarine will take one short and one long cruise. Peter is troubled by the thought of leaving Mary alone with the baby, especially since radioactive dust—traveling southward on global wind currents—is expected to reach Melbourne soon. Nevertheless, Peter accepts the position, knowing Mary would be mad if he did not.

Peter meets Dwight and invites the American captain to his home. Dwight accepts because he does not want to be stuck at the docks all weekend with only his thoughts and memories of his life in America. Peter returns home to tell his wife about his new assignment and the invitation he extended to Dwight. Mary gets upset that Peter invited the American to visit, remarking how painful it is for people from the Northern Hemisphere to go into people's homes and be reminded of what they have lost in the war. To distract Dwight from painful memories while he visits, Mary decides to have a party and invites her friend Moira Davidson. Moira is not particularly thrilled about meeting a widower from the Northern Hemisphere, but she accepts.