Near the end of this novel based on the murder of John F. Kennedy, D. M. Thomas has one of his characters, a psychologist, comment on the many thousands of people who are haunted by the assassination. For all of them, she writes, it “occupies a kind of dreamtime. Kennedy is dead, he is not dead. He is being taken back for burial at Arlington; he is flying on to Austin. A physicist said to me that those few seconds carried too great a burden of event, of shock, and it was as if that weight caused time to cave in, creating a vortex, a whirlwind, in which past, present and future, and reality and illusion, became confused.”

In effect, that psychologist is summarizing for us the argument of Flying In to Love. The novel itself is the vortex, in which Mr. Thomas dreams out the events of Nov. 22, 1963, again and again, each time from a different point of view and sometimes with a different result. In following the motorcade that departs from Dallas's Love Field, it is as if Mr. Thomas were running and rerunning the Zapruder film of the shooting, but changing the camera angles, moving backward and forward in time, adding characters, searching for a plot (in both meanings of the word), even imagining a world in which the assassination didn't succeed. In one sense, he is playing the what-if game long established in both historical and science fiction, but he is adding many more ifs than a less metafictionally inclined writer might dare.

Mr. Thomas, whose previous novels include The White Hotel and Ararat, assembles all of the expected cast—the Kennedys, the Johnsons, the Oswalds, J. Edgar Hoover—but he fictionalizes them with varying degrees of success. His Jacqueline Kennedy is the most sympathetic, truly in love with her husband, grieving over the baby she had recently lost, yet capable of great strength during the horror of the murder and its aftermath. Kennedy himself is a man incapable of deep affection for another individual, a man who can love only the faces in the crowds. Though there are moments when Mr. Thomas manages to make a reader feel sympathy for him, Kennedy remains too shallow, too wholly possessed by a gargantuan and amoral sexual appetite to seem fully credible, whether in a fantasy world or not. And Lyndon B. Johnson simply flops: a man who often seemed to be his own caricature can't be caricatured further, as Mr. Thomas attempts to do, without becoming merely a parody.

Of the invented characters, Sister Agnes, a young nun who was among those watching the motorcade, is the most interesting. When Kennedy stops his car and speaks to her, he sets in progress her lifelong obsession with him and his death an obsession that becomes a very complex kind of love. We stay with her through the stages of that love from the day of the assassination to the present, and see it manifest itself in dreams, fears, repressed lust, illness and finally bitterness. Sister Agnes serves Mr. Thomas well. She is both a soundly drawn character and a good vehicle for his exploration of the “vortex” caused by Kennedy's death, though her fixations on the martyred Jesus and on her father make some pretty heavy-handed symbolic connections.

Besides having difficulty creating convincing characters, Mr. Thomas sometimes runs into trouble wrapping his British tongue around the American idiom, especially the Texas variety. Although he is better at bilingualism than most writers who cross the Atlantic—in either direction—an American reader should come prepared to forgive, or not, some wooden dialogue and a few real boners. (“Nor me,” exclaims a witness at one point; the novel is heavily populated with “Texan men” and “Texan Democrats.”)

Despite its failings, however, Flying In to Love is worthwhile. Its greatest strength lies in Mr. Thomas's use of the assassination as the occasion for his book, rather than as the subject of it. He appears to accept the vast-conspiracy theory of the killing and, in a section of the novel he calls “Historical Fictions,” even expands it. Yet the conspiracy—or its unraveling—is not what ultimately interests him. Instead, he wants to puzzle out the need so many people feel for a conspiracy to explain the assassination, a “plot” to the events, so we can believe that reasons and causality do exist in the universe, even if they're hidden. Does history seem solid and fixed, Mr. Thomas wonders, only because we're seeing it in its apparently completed form, separate from ourselves? What does it mean to us, and to our dreams, to realize that we are history and, even if only like particles in chaos theory, we might somehow change it? What if, what if?

On one level, Mr. Thomas's game has been played often. (“It's a Wonderful Life” comes easily to mind.) But on the level at which Mr. Thomas plays, the intellectual difference is as profound as that between checkers and chess.