THIS IS THE WAY THE WORLD ENDS by James Morrow (Henry Holt: $18.95; 319 pp.)

“Thinking the unthinkable” is shorthand for how we contemplate the fact that humanity possesses weapons too awful to use, and yet, if precedent counts, it may literally end up using them.

The phrase is a logical contradiction, and another one can be tacked onto it. It is impossible to think the unthinkable, and it is also necessary.

And so, paying suitable tribute to Jonathan Schell and Freeman Dyson and their predecessors, we have this fictional fantasy by James Morrow. Faced with impossibility and necessity: Instead of thinking the unthinkable, Morrow capers with it.

It is a very serious caper, though antic. Morrow juggles his grim necessities winningly, and sometimes awkwardly. He drops a few. He lets his plot run away with him. Yet he achieves a lot. “This Is the Way the World Ends” is provocative most of the time, comic a good deal of the time, and it ends in blackness. Yet it is by no means black humor. Morrow doesn’t use absurdity to stave off despair, but embraces both of them. Having no answers but only prophecy, he is unafraid to feel pain and to make us feel it.

The story is in the form of an augury, made in the 16th Century by Nostradamus to a boy who wanders into his workshop. It concerns the year 1995; it has been illustrated on a set of painted glass slides by the necromancer’s friend, Leonardo da Vinci.

This framing device is whimsical and perhaps necessary; mainly, it gives Morrow the opportunity of drawing a crabby magician reminiscent of Merlin in “The Once and Future King.” He has more fun with it than we may, a situation that recurs from time to time through the book. It is not always unpleasing, perhaps because the spectacle of the author taking a break balances the somberness of his story.

In 1995, the U.S.-Soviet nuclear standoff is as frail as it is massive. Deterrence reigns, yet it is so fragile that our No. 1 growth industry is white survival-suits--blast-proof, burn-proof, radiation-proof, with built-in sanitary facilities and an automatic weapon. Men, women and children wear them. “America was becoming a safe, white country,” George Paxton notices.

George is innocent, reflective and candid. He is also, it turns out, Candide, although in a world where everything is for the worst. He lives in Wildgrove, Mass., where he carves epitaphs on tombstones. He loves his wife and adores his small daughter, Holly. He would like to buy Holly a survival suit, but can’t afford it. A mysterious old woman offers him one free, to be picked up in Boston, if he will compose a couple of epitaphs for her.

And as he drives back from Boston, the world ends, only three miles from home. So overstocked are the nuclear arsenals that the Soviet Union has targeted two missiles just for Wildgrove alone. Morrow’s description is extraordinary:

“The winter,” he writes, “which only that morning had smothered southern New England with snow, went away.” George is blasted out of his van into an adjoining stream. “Afloat on his back, he became driftwood. Blind. Eyeless. The wind hated him . . . the river hated him and so it sent him smashing into a log, crack. . . .”

George comes to, burned and sick with radiation, aboard a U.S. Navy submarine. A shore party has rescued him along with five other special passengers: an Air Force general, a nuclear scientist, a state department official, a disarmament negotiator and a fundamentalist preacher. They are headed for Antarctica where, their assumption goes, they are to conduct the continuing war.

George’s presence is a mystery. Is a tombstone carver part of the United States’ last-ditch battle plan? That would argue an unlikely realism on the part of the strategists. In fact, as we discover, the war is over, there are no survivors, the submarine crew is not Navy, and the voyage to Antarctica is for quite another purpose altogether.

It is the book’s central conceit, and a brilliant one, that the crew, and the community waiting for them in Antarctica, are not people at all. They are the Unadmitted. That is, they are those who might have been born had the 1995 generation not cut them off. And the submarine passengers are bound for a Nuremberg-like trial whose main charge is: murder of the future.

George, so clearly innocent, is there for the simple crime of complicity. Acquiring a survival suit is a fundamental acquiescence to war, a crucial step away from sanity.

The trial conducted by the Unadmitted--who themselves have only a year to survive--is a keenly intelligent, and only occasionally prosaic, fictionalization of the big questions. The tribunal president is a black woman who would have become one of our great Supreme Court justices had she been born. Another is a jurist who would have revitalized the World Court and won a Nobel Prize. A prosecution witness is a negotiator who--again, had he been born--would have negotiated total disarmament, guaranteed by space surveillance. And so on.

The defense, represented by a superb Unadmitted lawyer, gets full say. The reasonableness of the deterrence doctrine is laid out, along with its other-worldly convolutions. What about the Cuban missile crisis, the Berlin crisis--would they not have developed into bloodshed without the deterrent?

An Unadmitted general, who would have led a successful revolt in the Pentagon against nuclear weapons, is asked about his medals. They were for the Greco-Russian war, he replied. It was a conventional war, 200,000 American soldiers died in it. Deterrence gets its innings.

Against that, of course, is the end of the world and the death of the future. In a last-minute twist, it appears that deterrence had worked perfectly well. The radar screens had been misled by a flight of vultures.

“There will always be a vulture, gentlemen,” the tribunal president says. “When you booby-trap an entire planet, you cannot say ‘Non mea culpa’ if some faulty computer chip, misfiled war game, nuclear terrorist, would-be Napoleon, unmanageable crisis or incomprehensible event pulls the tripwire.” She sentences them all to hanging.

The book does not quite end. George escapes briefly with a woman doctor; perhaps they will begin a new humanity. It doesn’t work; destruction has been too final. The plot’s last convolutions are distractingly complex and somewhat forced. They have the advantage of providing more time for George whose sweetness, pain and originality are one of the book’s best strengths.

“This Is the Way the World Ends” is by no means a flawless fictionalization of final disaster, but it is an astute, highly engaging and finally moving one.