Soon after Harry Baldwin (Ray Milland), his wife Ann (Jean Hagen), their son Rick (Frankie Avalon), and daughter Karen (Mary Mitchell) leave suburban Los Angeles on a camping trip, the Baldwins note unusually bright light flashes coming from a great distance. Sporadic news reports on CONELRAD broadcasts hint at the start of a World War - which is confirmed as the Baldwins see a large mushroom cloud over what was Los Angeles. The family initially attempts to return to rescue Ann's mother near Los Angeles, but soon abandons these plans as panicked refugees from Los Angeles climb over one another to escape the fallout from the multiple nuclear explosions. Witnessing the very threads of society breaking down in front of them, Harry makes the decision that the family must make it to their secluded vacation spot in search of an isolated refuge. Along the way, they stop off to buy supplies — or, in the case of hardware store owner Ed Johnson (Richard Garland), take them by force when he won't accept a check — and extra gasoline. They also encounter three threatening young hoodlums, Carl (Richard Bakalyan), Mickey (Rex Holman), and Andy (Neil Nephew), on the road, but manage to drive them off.
After a harrowing journey, the Baldwins reach their destination and hole up in a cave to wait for order to be restored. They find that Johnson and his wife are their neighbors - but not for long. The three thugs show up and shoot them. A farming couple suffers the same fate and their teenage daughter, Marilyn (Joan Freeman) is kept as a sex slave. Mickey and Andy happen upon Karen and she is raped. With guns in hand, the Baldwin men fight back, killing the two murderers and freeing Marilyn. When Carl returns, he is killed as well, but Rick is seriously wounded.
With Marilyn's help, they get the young man to Doctor Strong (Willis Bouchey). The doctor does what he can, but the boy needs to get to an army hospital over a hundred miles (160 km) away or he will die. On their drive there, they run into a military patrol, scouting for the army that is reestablishing order. After a tense meeting, they are allowed to pass through.