Alas, Babylon is a novel bound to a specific time and place. It describes the peak of the Cold War between the United States and the Soviet Union, in the late 1950s and early 1960s. It tells the story of a war that did not happen, a destructive nuclear struggle that nearly destroys civilization in the United States. The arrival of war is attributed to two factors: Soviet Union's attempt to gain access to, and eventually dominate, the Mediterannean, and the idea of a "missile gap" that gives the Russians an advantage in missile technology and leads them to attempt a surprise attack on the United States.

But these political concerns are of secondary importance in Alas, Babylon. They enter the story in the person of Mark Bragg, who first explains the situation to his brother Randy, and after the war, the outside world appears only in the form of occasional radio broadcasts. But the novel is ultimately more interested in ordinary people than in international politics. The narrative focuses on the small town of Fort Repose, Florida, and how its residents deal with a remarkaable new situation. In this sense, this seemingly frightening and apocalyptic novel can actually be called optimistic. Its tone contrasts sharply with much of post-apocalyptic fiction, in which nuclear war either wipes out the entire population of earth or reduces humanity to a savage, Stone Age existence. Order collapses in Fort Repose, but over the course of the novel it is gradually restored, largely through cooperation and friendship between the main characters, all of whom are sympathetic figures. Racism, for example, is hinted at early in the novel, but never seems to become a problem after the war. Conflict between the main characters is almost nonexistent—danger comes from outside, from shadowy, one-dimensional drug addicts and gangsters. By the end of the novel, Fort Repose has weathered a number of crises involving food shortages, radiation poisoning, and crime, and it has become a reasonably pleasant place to live again. So pleasant, in fact, that when an offer arrives to helicopter some people to a safer part of the country, it is turned down, as the characters prefer to remain and continue rebuilding their home.

In addition to this optimistic view of the human spirit, Alas, Babylon also offers optimism on a smaller scale, in the person of the protagonist, Randy Bragg. When we first meet Randy, he is a likable man in decline, having failed in his attempt to run for political office. Since his defeat, he is sliding through life. He has no real job, and lives off the profits from his family land. He has taken to drinking before breakfast. In the aftermath of the nuclear war, however, Randy changes drastically, and his decision to take responsibility for his family and friends is a critical part of Fort Repose's climb toward order and civilization. With the heroic doctor Dan Gunn, Randy works first to build a safe, sustainable community for his family and neighbors on River Road, and then uses his authority as an officer in the Army Reserve to assume responsibility for law and order throughout Fort Repose, dealing with outlaws and keeping the peace. He becomes a kind of classic American hero—the reluctant sheriff who assumes responsibility for law and order in a frontier town.