The story details a comet colliding with Earth, an end to civilization, and the battle for the future. It encompasses the discovery of the comet, the Los Angeles social scene, and a cast of diverse characters whose fate is to be among the few who survive the massive cataclysm and the resulting tsunamis, plagues, famines, and battles among scavengers and cannibals.
When wealthy soap company heir and amateur astronomer Tim Hamner codiscovers a new comet, dubbed Hamner-Brown, documentary producer Harvey Randall persuades Hamner to have his family's company sponsor a television documentary series on the subject. Political lobbying by California Senator Arthur Jellison eventually gets a joint Apollo-Soyuz (docking with the second flight-worthy Skylab) mission into space to study the comet, dubbed "The Hammer" by popular media, which is expected to pass close to the Earth.
Despite assurances by the scientific community that a collision with Earth is extremely unlikely, many members of the public catch "Hammer Fever" and begin to hoard food and supplies and head for the hills in anticipation. Some are fueled with religious fervor by the evangelist Henry Armitage, who teaches that the arrival of the comet signals the End Times. Jellison quietly gathers supplies and retreats to his ranch in the foothills of the Sierra Nevada with his family and close aides.
Eventually, to the shock of scientists at JPL in Pasadena, who could not project the comet's path accurately enough because of the comet's constant outgassing and fragmentation, many pieces of the comet's nucleus impact around the world with devastating results, striking parts of Europe, Africa, the Gulf of Mexico, and both the Pacific and Atlantic Oceans. The strikes trigger several volcanoes and earthquakes around the world, including the San Andreas fault, heavily damaging the Southern California region and the rest of California, causing millions of casualties. Several of the fragments land in the ocean, causing further damage by the resulting tsunamis (which destroy major coastal cities around the world, including Los Angeles, killing millions) and long-term climate problems due to the massive quantities of vaporized seawater.
Immediately following the Hammer's impact, anticipating the coming ice age and the inevitable southward migration of survivors from Russia into Chinese territory, China launches a preemptive nuclear attack on major Russian cities. The Russians are able to respond in time and, with American assistance, China is effectively destroyed but Russia still suffers devastating losses.
Within hours of the comet strike hundreds of millions are dead and much of the world is left in ruin. As the survivors contend with weeks of nonstop rain, flooding destroys practically every dam and levee. Electrical power is lost and commerce breaks down as long-distance travel and communication become difficult or impossible. Civilization crumbles as people use the few remaining weapons to protect themselves from each other, leaving the search for food and security as top survival priorities.
Surviving "Hammerfall" is shown to be primarily a matter of random chance, with preparation being only a distant second factor. Hamner goes from being a dilettante astronomer to a determined survivor, with his new wife, Eileen. Randall shows true leadership abilities under fire, while Jellison and other land owners, farmers and ranchers become lords over their fiefdoms and the serfs they employ to provide labor, skills and security. Jellison's ranch forms the centerpost of these fiefs in the Sierra foothills, dubbed "the Stronghold," where he presides over a small population of survivors who struggle to maintain civilization. The tone of life after "Hammerfall" is one where those who do not have valuable skills for a world without power or civilization are relegated to being manual laborers, regardless of their socioeconomic status or profession before the Fall.
Shortwave radio, the only surviving means of mass communication, is eagerly monitored and reveals a chaotic situation after the Fall. Several people, including some who were previously officials of the former United States Government, claim, with varying levels of support, to be the new or currently acting President of the United States, while others are now self-proclaimed monarchs of various regions or areas. Many people subsist by looting former stores and by catching rats and various fish, especially the now-plentiful giant carp, which are mostly either former pet goldfish or their progeny, engorged by the massive supply of food available to them, primarily in the form of human and animal corpses. Some advanced technical knowledge was maintained by the means of the preservation of a collection of books which had been wrapped in impermeable plastic and submerged in a septic tank shortly after Hammerfall and later retrieved by a JPL scientist who realized their potential value and likely scarcity in a post-Hammer world.
While doctors and farmers are still valuable, lawyers, executives and many other professionals are unnecessary, but if civilization is to be rebuilt, scientific knowledge is the most valuable skill of all. Soldiers and police are diminished in status, and provide security alongside gang members and bikers, both within the Stronghold and within the New Brotherhood Army. This latter group forms the legions of Reverend Henry Armitage, who managed to take control of the remnants of a former United States Army unit which had largely survived the Fall due to its being on maneuvers in the Sierra Nevada mountains, at altitudes high enough to survive the flooding. Armitage integrates the soldiers and his pre-existing band of followers, initiating them into cannibalism to shame them into loyalty, and maintains even more loyalty through a strict egalitarianism, where racial, social, or other prejudice is severely punished.
Armitage's charisma and his followers' strict, military-derived discipline makes the New Brotherhood Army into a formidable force. Jellison's stronghold is located slightly east or northeast of Springville, California, where the North Fork and the Middle Fork of the Tule River meet. West of this stronghold, the city of Porterville has been destroyed by the collapse of the dam at Lake Success. Massive and sustained rainfall has turned the former San Joaquin Valley into a swampy lake. Other small enclaves of civilization exist in this area until the band of cannibalistic zealots led by Reverend Armitage and his army of heavily armed soldiers begin a rampage through the area, culminating in a series of battles with the inhabitants of the Stronghold. Jellison's force prevails, saving the Stronghold while also defending a nuclear power plant nearby, thus preserving a supply of electric power needed to rebuild civilization.
Judith T. Yamamoto in her review for the Library Journal said that the novel was full of "good, solid science, a gigantic but well developed and coordinated cast of characters, and about a megaton of suspenseful excitement." Her one negative comment was that the pro-technology pitch might turn off some readers but "all in all it's a good book, if not a great one." Lucifer's Hammer received a nomination for the Hugo Award for Best Novel in 1978.