It would begin with an announcer breaking in on television and radio programs: ''Although no attack has yet been launched upon the United States, the president has directed the governor to order the immediate evacuation of areas of the state which are considered to be probable targets if a nuclear attack occurs.''
If you live in one of the nation's large industrial cities, near an important military base, or close to a key defense plant, you would know that you were a probable target. That's why the federal, state, and local governments have come up with plans to try to evacuate everyone from target areas to rural counties where they might have a greater chance to survive a nuclear exchange. Those plans call for moving about 150 million Americans in 72 hours.
But the plans have become controversial in themselves. Some civil defense officials say they won't work, and they could cause chaos if no attack is launched. Peace groups have condemned them for giving the public a false sense that they would be safe in a nuclear war.
''The plan is entirely unrealistic, and it conveys an impression that something useful is being done to prepare for a nuclear war,'' says retired Adm. John Marshall Lee, who had worked in strategic planning for NATO and the US Defense Department. ''The government is saying we have a plan and we can face a nuclear war, and that's not true.''
While federal and state emergency planners agree that the imminent threat of a nuclear war would create havoc, they say that having an evacuation plan will save at least some lives and is far superior to having no plan at all.
''The intellectual approach now is to say we'll all sit around and hold hands when the bomb comes because there is no escape,'' says Chip Hultquist, who put together Florida's recently completed evacuation plan. ''But there's a basic urge for survival, and if it appeared an attack were coming, the survival reaction would take over, and people would do what they could to get out of the way. We're just helping them.''
Planning for a nuclear war is much the same as planning for other major catastrophies,'' says Russell Clanahan, of the Federal Emergency Management Administration (FEMA), which oversees civil defense planning. The plan for nuclear war defense also helps earthquake and hurricane planning.
But opponents to nuclear defense planning say it will not work.
They say the plan needs five days' warning before an attack is launched, which they contend is unrealistic. It also depends on thousands of people being willing to stay behind as emergency workers, on everyone behaving rationally, and on food and other essential resources to be delivered orderly to the ''host'' area.
''But in fact, it would be chaos,'' Admiral Lee says.
In an all-out nuclear war, almost everything in the northern hemisphere would be contaminated with radiation, opponents say, and recent studies have strongly suggested that explosions from a nuclear exchange would create a ''nuclear winter'' of freezing temperatures that would obliterate life on the planet.
By having a civil defense plan, they say, government officials may think they could win a nuclear war, and the general population could be lulled into believing it's true.
If an evacuation were ordered, they say, the Soviets might be forced to attack because they would believe the US was about to attack them.
Officials who drew up the plan concede that it is not perfect, but they insist that it is better than no plan at all.
''I definitely think it will work,'' says Florida's Hultquist. ''We can't expect to save 100 percent of the people, but we have an obligation to protect the health and safety of the population, so we have to do what we can.''
It is not unrealistic to expect to have five days' notice for an evacuation, FEMA's Clanahan says. The Soviets would not attack until they had evacuated their own population, he says, and the president would not begin evacuation unless the Soviets had begun theirs.
And no president is going to believe he can launch a nuclear war just because he has a civil defense plan, Mr. Hutlquist says. And some studies, he says, have questioned the ''nuclear winter.''
Government officials in some states don't agree. The legislators in California, Maryland, and Massachusetts have voted not to spend any money on planning for nuclear war civil defense. Last December, the governor of New Mexico wrote President Reagan and said he would not support any planning.
Other states are considering joining the planning boycott, several cities and counties have followed suit.
''We think it's a travesty sending any money for crisis relocation,'' says an aide to Democratic California Senate majority leader John Garamendi. ''It's a sham to think we can survive any nuclear attack by putting people on cars and buses and sending them into the country. It works against arms control, and this is the only action the state legislator can take to make that point.''