A nuclear war has destroyed the Northern Hemisphere, and radioactive dust is drifting south at a steady rate. In less than one year, the radioactive cloud will reach those living in southern Australia.
In Melbourne, the southernmost large city in the world, Peter Holmes, a Lieutenant Commander in the Australian navy, is appointed as a liaison officer to the U.S.S. Scorpion, an American nuclear submarine captained by Commander Dwight Towers. Peter accepts the position, even though he is troubled by the thought of leaving his wife, Mary, alone with their baby daughter, Jennifer, especially when the radiation is likely to reach Melbourne in only six months.
Peter invites Dwight to spend the weekend at his home in Falmouth, a suburb of Melbourne. There, Dwight meets Moira Davidson, a young woman who drinks heavily to forget her despair about the fact that her impending death means she will never have the future about which she has dreamed. On Sunday, Dwight goes to church alone to think about his family in America. Although he knows everyone in the Northern Hemisphere is dead, he still thinks of them as if they are alive and waiting for his return to Connecticut.
The next day, John Osborne, a civilian scientist, joins the crew of the Scorpion. Dwight invites Moira to tour the submarine and then go dancing. A day later, the submarine sets out for an eleven-day cruise up the coast of Australia to look for traces of life. The only living creature they see on the entire voyage is a single dog.
On board, the crew talks about the war. No one has written a history of the war, and no one really even knows how it started or escalated. They know that Albania started an Arab-Israeli War, which led to a Russia-NATO war, which led to a Russo-Chinese war. When the Scorpion returns, they report to their superiors about their disappointing trip. A crewmember gets the measles, and Peter's wife worries that baby Jennifer will get sick.
Dwight and Moira spend more time together and become good friends. Dwight talks about his family as if they are still alive. Moira falls in love with Dwight, but she realizes he will remain loyal to his wife. Dwight visits Moira's family's farm, and he is delighted to see trees from the Northern Hemisphere. Moira's father, like many other characters in the novel, cannot accept his inevitable demise, and he is still preparing his farm for future years. Peter and Mary are also spending time planning their garden, planting flowers that will not bloom until after the couple is dead.
When Dwight and Peter return to Melbourne, they find out their next mission is to investigate the Jorgensen effect, a controversial theory that claims that snow and rain will cleanse the atmosphere so the radiation will never reach southern Australia. They are also instructed to investigate the source of a mysterious, intermittent radio signal that has been coming from the Seattle area. After the briefing, John shows Peter his red Ferrari. John has bought the car because he has always dreamed about racing and now has the opportunity.
Before Peter goes on the submarine expedition, he tells Mary that the radiation sickness might reach her and Jennifer while he is away. He tells Mary that she may have to euthanize Jennifer with a lethal injection so that the baby will not die a slow, painful death from radiation sickness. Mary becomes hysterical, accusing Peter of wanting to murder her daughter, but she later apologizes for not accepting their inevitable fate.
Meanwhile, Moira surprises Dwight by telling him she is taking secretarial courses and drinking much less than she used to. Dwight buys a fishing rod for his son and an expensive bracelet for his wife. He desperately wants to give a pogo stick to his daughter, but all the stores are sold out, so Moira promises to find one for him while he is away.
The Scorpion sets out for the west coast of the United States. When they are just offshore of a small town near Seattle, Yeoman Swain, an American crewmember, jumps ship and swims ashore, knowing that the radiation will kill him, but wanting to die in his hometown. The sub continues up the coast and investigates the mysterious radio signals coming from the Seattle area. Lieutenant Sunderstrom goes onshore in a protective suit and finds out the mysterious signal—a source of hope back in Australia—is merely being generated by a broken windowpane that has been teetering on top of the transmitting button whenever the wind blows.
Dwight comes down with a fever when he returns to Australia, and he stays at Moira's farm to recuperate. Moira gives Dwight a pogo stick to give as a gift to his daughter. The radiation is now supposed to arrive in Melbourne in August, and the residents of the city are beginning to slow down and do what they really want to do in their final days. John races his Ferrari in the qualifying heats for the Australian Grand Prix. Many drivers take big risks and are killed in the race, but many consider such a death preferable to radiation sickness. John eventually wins the Grand Prix. Peter and Mary spend time tending to their garden, while Moira and Dwight go fishing in the mountains.
By the end of August, everyone becomes ill from the radiation. John takes a cyanide pill while sitting in his Ferrari. Peter euthanizes Jennifer, and then he and Mary take pills together in bed. Dwight dies while performing what he sees as his final duty—sinking the Scorpion in international waters, going down with the ship. Moira had hoped to go onboard with Dwight, but he refused her, saying that the U.S. Navy would not approve. Instead, Moira drives to a cliff overlooking the ocean to watch the submarine go out to sea. Sitting in her car, watching the ship go down, she takes her final pill with a shot of brandy.