What with all the talk about war and the nuclear balance in this presidential campaign, the message of "Nuclear Nightmares" (tonight on Channel 26 at 8) could hardly be more timely:

We are armed to the teeth; so are the Soviets; about a half-dozen other countries either have the bomb or are on the way to getting it; and it is all very, very dangerous.

To say that all this is obvious is not to belittle its importance. Watching this program is like listening to a well-delivered sermon. Predictable. Earnest. Depressing. But ultimately not as stirring as it should be, given the subject. Ninety minutes of prime time on the subject of scenarios for nuclear holocaust doesn't happen every day and should not be dull.

Alas, "Nuclear Nightmares" is dull.

Peter Ustinov is the host and traverses the world, juxtaposing images of tranquil normalcy (a street scene in Paris, a fountain in Geneva) with footage of military hardware and tense border scenes. We see bristling Soviet armor, advanced American weaponry, mushroom clouds. We hear Ustinov ask, "How much bang for a buck? How much rubble for a ruble?"

Interspersed are fantasies from an underground bunker of the future in which Ustinov delivers monologues on how the terrible final war began. One scenario starts on the East German frontier as an unexpected Soviet advance into West Germany triggers a Nato response. Another flashpoint is a terrorist nuclear attack on Jerusalem. Yet another is a Soviet-Chinese war that finally envelops the U.S. as well. All are terribly plausible.

In fact, nothing about "Nuclear Nightmares" can be faulted on content. The assessment of what Ustinov calls "cosmic luunacy . . . as government policy" is absolutely sound. The notion that the United States, which already has the capacity to destroy the world many times over, feels itself vulnerable is bizarre but undeniable. The idea that the Soviet Union, with the greatest conventional forces ever amassed, believes itself in peril, is equally absurd. And equally true.

What is missing from "Nuclear Nightmares" (intended, I suspect, as an antidote to more hawkish PBS offerings lately) is real analysis of the nuclear balance. We get truisms about nuclear dangers instead of some hard new reporting about why billions are being wasted on defense systems or why the U.S. and U.S.S.R. so seriously misread each others' motives.

There is a case to be made for pressing ahead with serious arms-control talks, reviving SALT and bearing down on the sagging effort to stop the proliferation of nuclear weapons technology. But this program doesn't make it. Another problem is that the show was filmed (a collaboration of the BBC and WNET in New York) over a year ago and seems dated. This was before the Soviet invasion of Afghanistan, the taking of our diplomats as hostages in Iran and the nomination of Ronald Reagan for president. All have had a bearing on the argument and needed to be factored in.

"Nuclear Nightmares" set its sights very high and then didn't or couldn't deliver.