Randy (Randolph) Bragg lives in the small Central Florida town of Fort Repose and appears to be drifting down a somewhat aimless path in life when his older brother, Colonel Mark Bragg, an Air Force Intelligence officer, sends a telegraph ending in the words "Alas, Babylon," a pre-established code between the two brothers warning of impending disaster. Mark is flying his family down to Fort Repose for their protection while he stays at Offutt Air Force Base in Omaha, Nebraska.
Soon after, an American fighter pilot attempting to intercept an enemy plane over the Mediterranean inadvertently fires an AIM-9 Sidewinder heat-seeking missile that goes off course and hits an ammunition depot in Latakia, Syria, resulting in a large explosion. This event becomes the apparent casus belli for the Soviet Union to launch a nuclear strike against the United States and its allies. Early the following morning, Soviet missiles arrive from over the Arctic, as well as from submarines, while American missiles are sent in response. Randy and his house guests are awakened by shaking due to bombing of various air force bases and naval air stations across Florida; one explosion temporarily blinds Peyton, Randy's niece.
Things are chaotic at first: Tourists are trapped in their hotels, telegraph lines fail to work, the use of the CONELRAD radio system exposes its weaknesses, convicts escape from jails and prisons, and a run on the bank results in the bank's closing. Randy organizes his immediate neighbors to provide housing, food, and water for themselves.
Over the months, news trickles in exclusively through the radio. Most of the government on both sides has been eliminated. The current American president, Josephine Vanbruuker-Brown, was formerly the Secretary of Health, Education and Welfare and in an address to the remaining nation, she notes that she is one of the most junior cabinet officials, as everyone above her in the line of succession is dead. Having been active as a part-time Army Reserve officer before the Soviet attack, Randy organizes a community self-defense team against bandits and works to rid the community of radioactive jewelry smuggled into Fort Repose from the ruins of Miami.
The following year, Air Force helicopters make contact with Fort Repose. While 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 as it is now the recipient of aid from third-world countries such as Brazil and Venezuela.