Randy Bragg lives an aimless life in the small Central Florida town of Fort Repose. His older brother, Colonel Mark Bragg, an Air Force Intelligence officer, sends a telegram ending in the words, "Alas, Babylon", a pre-established code between the brothers warning of impending disaster. Mark flies his family down to Fort Repose for their protection while he stays at Strategic Air Command headquarters at Offutt Air Force Base in Omaha, Nebraska.
Soon afterward, a U.S. fighter pilot, attempting to intercept an enemy plane over the Mediterranean, inadvertently destroys an ammunition depot in Latakia, Syria, resulting in a large explosion. This event escalates cold war tensions to the point the Soviet Union launches a full-scale nuclear strike against the United States and its allies. U.S. missiles, with Mark as a witness, are sent in retaliation. Randy and his guests awake to the shaking from the bombing of nearby military bases; one explosion temporarily blinds Peyton, Randy's niece.
At first, things are chaotic: tourists are trapped in their hotels, communication lines fail to work, the CONELRAD radio system barely operates, convicts escape from prisons and a run on the banks results in currency being basically worthless.
As the months wear on, news trickles in by radio. Most of the government has been eliminated, with the current U.S. president, Josephine Vanbruuker-Brown, being the former Secretary of Health, Education and Welfare. Since Randy was an Army Reserve officer before the Soviet attack, he organizes a community self-defense team against bandits and tries to rid the community of radioactive jewelry taken into Fort Repose from the radioactive ruins of Miami.
The following year, Air Force helicopters arrive at Fort Repose. When they offer to evacuate the residents from Florida, which is considered a "contaminated zone", the residents choose to stay. It is revealed that the United States won the war, but at a tremendous cost. It is now receiving aid from third-world countries, such as Brazil and Venezuela.