First, the (possibly) bad news: If you’re expecting Stephen King to provide an alternative history of what America would have been like had John F. Kennedy not been assassinated in Dallas, put those expectations aside.

Not until 800 pages have gone by in “11/22/63” does King offer up an account of the world as it might have been, and even then it has a cursory, I’m-doing-this-because-I have-to feel to it. This does not belong on the What If? shelf that has given us the Nazis-win works of Robert Harris (“Fatherland”) and Philip K. Dick (“The Man in the High Castle”), or the Charles Lindbergh presidency of Philip Roth (“The Plot Against America”).

Now, the (mostly) good news. That is not what King is aiming at.

He is, instead, offering a tale richly layered with the pleasures we’ve come to expect: characters of good heart and wounded lives, whose adventures into the fantastic are made plausible because they are anchored in reality, in the conversations and sense of place that take us effortlessly into the story.

The suspension of disbelief required here happens almost before the book begins. In this case, it begins in Lisbon Falls, Maine (of course), in a diner whose proprietor, Al Templeton, summons Jake Epping for an urgent meeting. Somehow, overnight, Al has aged years and contracted a fatal illness. But it hasn’t really happened overnight; in fact, he has been gone more than four years and has traveled through a time portal that connects the present to Sept. 9, 1958.

‘11/22/63: A Novel’ by Stephen King (Scribner. 849 pp. $35) (Scribner)

Al, who began with the modest goal of buying hamburger meat at 50-year-old prices, had taken on a very different task before illness sent him back home to the present: trying to prevent the murder of John F. Kennedy.

“If you ever wanted to change the world,” Al tells Jake, “this is your chance. Save Kennedy, save his brother. Save Martin Luther King. Stop the race riots. Stop Vietnam, maybe. . . . You could save millions of lives.”

In Epping, an English teacher, Al has found the right guy to finish the mission he’s too sick to complete. Divorced and childless, Epping has his own, less-cosmic reason to undo the past: preventing a horrific act of violence that still darkens the life of a friend in town.

So back he goes to a world where everyone smokes, where the food is tastier, the people friendlier, where the gunshots that changed America are more than five years away.

But Jake quickly discovers disturbing signs that the past is asserting itself against the threat of change. He meets people with the same names and hears the same words spoken in different places, in different “pasts.” He has with him a CliffsNotes version of the history he is about to live (in part so he can support himself by placing wagers on sporting events — a practice that exacts a terrible price). He settles down as a teacher in a small Texas town, falls in love, and all the while tracks the movements of Lee Harvey Oswald. (Al has made it clear that Jake must be positive that Oswald is acting on his own, and not as part of some larger conspiracy.)

King has done a prodigious amount of research here, which is both a strength and a weakness of the book. The depiction of Oswald and his family — his beautiful Russian wife, his overbearing mother — rings true, and Oswald’s motivation becomes clear: He’s an angry, twisted man determined to be “great.” But the piling on of detail after detail slows the pace and the pull of the story. In contrast to very long books like “The Stand” and “Under the Dome,” this work could have benefited from some serious paring.

Very much in evidence, however, are the memorable characters who populate so much of King’s work — people who touch us viscerally and for whom we root. (In fact, for me, watching Jake and the school librarian he falls in love with nurture their students was more absorbing than watching Jake keep tabs on ­Oswald.) And there is that powerful sense of place: in the stores, songs, clothes and cars, in the details that make this fantasy seem plausible.

When at last Jake races through the streets of Dallas on Nov. 22, King’s storytelling skills kick into overdrive. There are echoes of a hundred chase movies, but in this case the nemesis is the past itself, hurling obstacle after obstacle in the path of a man trying to avert the killing he knows is minutes away.

And while the unintended consequences of Jake’s journey seem to me too casually rendered — summed up by the old ad line that “it’s not nice to fool Mother Nature” — the ending is redeemed by a poignant reunion of sorts with the love of his life . . . make that, one of his lives.

We are, in sum, reminded again that in Stephen King, we have proof that (as JFK himself once put it) “life is unfair.” He is not only as famous and wealthy a writer as any of his time; his work suggests that if a time traveler found a portal to the 22nd ­century and looked for the authors of today still being read tomorrow, Stephen King would be one of them.

Greenfield has been a correspondent and political analyst for ABC, CNN and CBS and now serves as co-host of PBS’s “Need to Know.” The paperback edition of his latest book, “Then Everything Changed,” will be out in February.



By Stephen King

Scribner. 849 pp. $35