By Allen Appel

Doubleday. 405 pp. $19.95

TO REACH back and change the past -- surely that is one of the most enduring and endearing impulses of the modern mind. Science fiction has attacked this great thought-experiment with myriad purposes and results. Here Allen Appel follows up on his successful novels, Time After Time and Twice Upon a Time, which deal with historian Alex Balfour's lurches from era to era in pursuit of moral meaning and simple survival. Till the End of Time is an engaging read, zesty and vivid, a lively page-turner.

Yet it is not without moral purpose, as far too many popular novels are -- and seemingly these days, must be. Appel is after larger game than the reader's attention span.

As in the earlier books, Alex moves through time without scientific prop or artifice, because of a genetic quirk. In this case he swings back and forth between our complex era and the seemingly simple moral landscape of World War II. The contrast between our cynical and expedient time and the clear demands of the greatest of all wars is striking.

Today we fear nuclear war somewhat, but find our greatest enigmas in environmental and other issues. In Till the End of Time Appel's hero has a chance to make the opening of the nuclear age less bloody. That his ideas would be turned into a weapon used against civilians horrifies the aging Einstein. He urges time-traveler Alex to make Roosevelt set down guidelines for use of the bomb, well before 1945.

Should Alex try to change history? "And yet, didn't he have to make the attempt, even though the outcome was doubtful? That part of Einstein's philosophy had to be correct; a moral man must attempt moral change."

Even though Alex gets an interview with FDR, the wily politician derails his course into a fact-finding mission to the South Pacific. This admits us into the real world of the Big War, with some splendid action writing. We get the full tour, with well-considered meetings -- JFK as uncertain skipper of PT-109, "Wild Bill" Donovan as manipulator supreme. FDR stays in touch with messages relayed through notables, particularly a wanton Betty Grable, who supplies some welcome steam.

Appel neatly contrasts the certainties of that time with our own present. His lady love, mired in the journalism of 1990, pursues a parallel plot involving Japanese germ warfare. Her Washington, contrasting vividly with FDR's, is convincing: "Molly examined the heavy, greasy food and understood why most of Washington's government workers seemed to harbor an air of gloom and defeat."

But the miasma of our age is strangely neutral, as though the issues of that distant war were mere movie material: "The Germans have been able to accept the enormity of their crimes, to dwell on them at a national level, but the Japanese will admit very little. And the Holocaust, as terrible as it was, is over. Germ warfare continues, or at least the development side does." Indeed, Iraq's use of mustard gas reminds us that there is still no true international moral consensus.

This novel seems at first to be a simple action gambol, but it raises issues seldom treated in our press. The Japanese did carry out terrible experiments on captives. They did allow to die or outright killed half their prisoners of war, while the Germans lost a few percent. Yet the power of the Nazi imagery is such that we ascribe the blackest role to the Germans, and forget the Japanese. We wring our hands over our use of the atom bomb, though in fact our fire bombings of Dresden and Tokyo killed more. Further, the Japanese understood this long ago. They had an atomic bomb project. "They would in fact hide the possibility that they had ever even attempted such a thing, just as they hid the fact that Hirohito was an active participant in Japan's war plans."

True enough, and the novel capitalizes on this, framing its conclusion at Hiroshima, where the author places the Japanese A-bomb project. This serves some dramatic purpose (though in fact the work was in Tokyo), which however fails to render the climax as powerful as the earlier material.

Should the U.S. have foresworn the bomb, despite the deaths? Or was that clear demonstration of power a crucial object lesson? What would Einstein have said, if he had seen how it has all turned out (so far)? These are interesting questions, but alas, Till the End of Time, for all its fun, avoids them to its loss.

Gregory Benford is a professor of physics at the University of California, Irvine. His most recent book, with Arthur C. Clarke, is "Beyond the Fall of Night."