Tim Hamner arrives at a party thrown by Julia and George Sutter to announce that a comet he discovered has been authenticated and named the Hamner-Brown Comet. The "Brown" is a youngster who reported his own sighting about the same time. Tim explains that he found it first but waited for some time to be certain that it really was a comet before making the report.

Over the coming months, many people will worry that the comet might strike Earth and some prepare for end-of-the-world scenarios. Tim soon meets Harvey Randall, a documentary film maker who covers the scientific data regarding the Hamner-Brown. Harvey professes to believe the experts who say the chances of a comet strike are minimal but makes some preparations anyway. When the strike does occur, thieves hit the Randall home, killing Harvey's wife, Loretta, and taking all the supplies Harvey had gathered. Harvey's son, Andy, is in the mountains on a hiking trip with the Randall's neighbor, Gordie Vance. As Harvey leaves the city in the company of his colleague Mark Czescu and Mark's girlfriend, Joanne, they encounter Gordie's wife, Marie, and take her with them.

In the mountains, Andy and Gordie kept the troop of scouts safe, killed a group of bikers who took girls hostage, and declined the offer to go into a safer community in favor of remaining in this lifestyle. Nearby, Senator Arthur Jellison has gathered some supplies for a stronghold, though not enough that people would have noticed that he's expecting the comet to strike. While he turns away many, those inside band together. Jellison's word is law, to the point that he holds the power to order executions for those who hoard goods rather than putting them into a community fund for the common good.

When the comet does hit, some who were prepared are already off the coastal regions. Tsunamis hit within the hour and earthquakes do massive damage. Four astronauts - two Americans and two Russians - watch in space as China fires on Russia, Russia fires back and the United States joins in Russia's defense. The astronauts come to Earth in the Russian shuttle because the American ship is damaged. They join in the fight of those living on Jellison's ranch as thieves join to form a band of cannibals and attack those who are merely trying to survive. The stronghold created by Jellison soon becomes home to a group forced to fight to survive. Many are turned away because the meager resources within aren't sufficient to sustain them through what is going to be a difficult winter. Those with skills that can help the community thrive are welcomed, including the astrophysicist in possession of a "how to" book that covers things from napalm to insulin.

Tim, Harvey and others will eventually come together for another common cause - to protect a nuclear power plant that could give them a boost toward re-establishing a civilized country.