Alas, Babylon (by Pat Frank) takes place in the late 1950s, in the (fictional) idyllic all-American small town of Fort Repose, Florida, during the peak of the Cold War between the US and the Soviet Union. Randy Bragg, the main character, receives an urgent message from his big brother Mark Bragg, an Air Force officer, that there's an inevitable nuclear attack coming, and soon. Mark sends his wife (Helen Bragg) and two children (Ben Franklin Bragg and Peyton Bragg) to live with Randy, a bachelor, because he believes that Mark's wife and children will be safer away from their home in Omaha, Nebraska, which is near an Air Force base. After he gets Mark's message, Randy immediately sets about preparing for the inevitable by stockpiling food and other necessities. He also makes sure to warn the Henrys, an African-American family that lives nearby, as well as his girlfriend, Lib McGovern. The morning after Helen and the kids arrive, atomic bombs are dropped on all of the U.S.'s major cities. This was all a part of Russia's attack on the U.S. All of Washington D.C.. is obliterated, including the White House, leaving only a low-level cabinet secretary to lead the nation.
Once Orlando, Florida is nuked, all of Fort Repose's power supply is cut off, including most of America's, too, leaving their rural small town completely isolated from the rest of the U.S. It also leaves all of their current technology and conveniences useless. The town erupts into chaos, leaving supplies of food and gas almost nonexistent. The local doctor and Randy's best friend. Dr. Dan Gunn has his clinic ransacked by drug addicts. People die from heart attacks, and others commit suicide, most notably the town mayor and Edgar Quisenberry, President of the Fort Repose bank.
As the story progresses, Randy slowly invites people to live with him, despite his internal conflict about it. He invites his neighbors, Dr. Dan Gunn, Lib McGovern, and her family, the McGoverns, to live with him in his spacious house. They manage to create a system of order, as well as taking the initiative to share what few resources they have with each other and to care for one another, and this cooperation is ultimately what keeps them all alive, healthy, and safe.
They rely heavily on sharing crops of oranges, a safe artisan water supply, and all the skills that each person can contribute. Dr. Dan Gunn continues making house calls with Randy's car, making his clients pay him with a gallon of gasoline instead of cash or check, which has long since become essentially worthless. They trade for the things that they need with other people in town, some long-time residents of Fort Repose, and others just lost wanderers passing through.
At first it had seemed that they might've escaped radiation fallout, but then three Fort Repose residents are diagnosed with radiation poisoning. It turns out that it's from contaminated jewelry from a nearby city. In order to deal with this, Dan and Randy bury the jewelry in a lead-lined coffin with Porky Logan's corpse. Randy has to threaten other townsmen with his gun so that they'd help with the burial.
Randy has proven himself an amazing leader so far, but when an announcement is made over the radio that allows all former Army Reserve officers (Randy was one in the past) to take full responsibility for the safety of their town, giving Randy official authority over all the denizens of Fort Repose. The denizens of Fort Repose comes to respect Randy more and more as he slowly develops into a true leader, worthy of protecting his town. Randy's relationship with his girlfriend develops further, and they get married on Easter Sunday, mere months after the bombs, because Randy realizes that as the new leader of Fort Repose, he has the power to issue marriage licenses.
Crisis strikes yet again when Dan Gunn is making a house call; while driving home, he's robbed by highwaymen who take his car and his medical bag. Randy is furious, and makes a plan to track down each and every one of the highwaymen and kill them as both a punishment and as a preventative measure to ensure that they don't do the same, or worse, to everyone else, especially in Fort Repose or even in any of the surrounding roads and towns.
They use a grocery truck as bait to lure them out and manage to kill two and capture another, but in the process, Randy's neighbor and trusted friend Malachai Henry is killed, dying a slow death. Life goes on, and the town struggles with food shortages, most importantly a loss of salt. By reading the journal of his ancestor, the founder of Fort Repose itself, Randy locates a beach downriver made almost entirely out of salt (instead of sand) and with an innumerable amount of crabs that they could catch to bring back and eat.
At long last, planes begin to fly over Fort Repose once again; and a helicopter lands containing members of a decontamination crew, who declare that Fort Repose is now completely free of any and all radiation. They also say that a little while before all of that had happened, America had finally managed to beat Russia in the war, but not without millions of casualties.
Because of the complete lack of electronics, the denizens of Fort Repose don't discover this until that very moment. One of the members of the decontamination crew is an old friend of Randy's, who informs Helen that her husband Mark (Randy's brother) tragically didn't survive the bombing, which leaves Helen free to admit her feelings for Dan Gunn. Randy makes sure to ask that the decontamination team brings more medical supplies. The decontamination team offers them an airlift out of Fort Repose, but they all agree that they want to stay in the town, for better or for worse, with the town and the people that they've come to love.