Stephen King is a great American storyteller  though not, perhaps, quite so great as some of our more posturing critics, keen to prove how "down" they are with the masses, would have you believe. For in 11.22.63, his 49th novel, we see him fatally stymied by his own plot. In love with his conceit  he has written a time-travel story in which a man can move between 2011 and 1958 at will  King has delivered a self-indulgent book that is too long (a whopping 740 pages), too complicated and too barmy for words. Narrative tension, ordinarily his greatest skill, has been ruthlessly sacrificed on the altar of pedantry. I had never thought to hear myself call King boring; as a judgment, it's like saying that PG Wodehouse isn't funny. But there you have it: I wouldn't have finished 11.22.63 if I hadn't been reviewing it.

Jake Epping lives in Lisbon Falls, Maine, where he teaches high-school English and likes to hang out in a diner where prices are so low, the rumour is that the hamburgers are made of cat rather than best chuck beef. One day, however, having been summoned to the diner by a mysterious phone call from its owner, Al, he discovers the real reason why the menu is so replete with bargains: the meat is bought at 1958 prices. Al, you see, has discovered a portal to 1958 at the back of his pantry, and has been making regular journeys there for some time. No one notices his absence because these trips last for only two minutes in 2011, even if he stays there for years. The rules surrounding his passage to 1958 are complicated and contradictory. But of one thing he is absolutely sure. Every visit is a "reset". Whatever he accomplished last time is instantly erased the minute he returns.

Al has called on the younger Jake  a man handily divorced and childless  because he is dying. He will make no more visits to 1958. What's more, the lease on his diner is up, and when the landlord takes over, perhaps the portal will disappear for ever. Time, then, is of the essence. Al wants Jake to go back to 1958, and stick around until 1963. He hands him some notes: an exhaustive account of the movements of one Lee Harvey Oswald during this period. Yes, Al wants Jake to prevent Oswald shooting and killing President John F Kennedy on 22 November 1963 in Dallas, Texas. He planned on doing the job himself before the cancer. But now this intimidating baton must be passed.

Jake experiments with the portal, making a brief visit to 1958, where he drinks a root beer so delicious, it seems to mess with his brain. When he returns he hints that he will take on the job. First, though, he wants to try out another scenario  see how the land lies, both in 1958 and 2011. Before Oswald, he will attempt to stop another killer: the father of Harry Dunning, the janitor at his school. Harry's mother and siblings were murdered by his father with a sledgehammer, a past Jake only discovered when Harry wrote an essay on "the day that changed my life" in an adult education programme that Jake was teaching.

This mission is the first of a series of tedious and plodding false starts and digressions. Our hero doesn't reach Dallas until somewhere around page 260, and even when he does, he finds that he doesn't like it much, whereupon he promptly moves to an idealised small town called Jodie, takes a job teaching, inspires his jock students to get involved in the school play, and falls in love with the school librarian. This long, rather cheesy section is plainly an excuse for nostalgia (King was 11 in 1958), though this doesn't mean that it is atmospheric. Lindy hops, Chevrolets and thick cigarette smoke apart, King only goes through the motions; Mad Men, rich and complex, this isn't, and I never believed in his 1958 any more than I believed in his portal. Even Jake's shock and indignation at discovering that, in segregated America, the public lavatory for black men and women is just a plank of wood placed over a stream felt tinny and manufactured.

I suppose I'd better not go into too much detail when it comes to events in the Texas School Book Depository (I'm doing my best to put you off, but there's no accounting for taste). Suffice it to say that Jake does tear himself away from Jodie in time to meet the attention-seeking Oswald as he returns from the Soviet Union, his mothership of choice. Does King believe that history would be substantially different had JFK lived on that fateful day? Yes, though not necessarily in ways you might have expected (and Paul McCartney, among several others, will pay the price).

King, in as much as his novel is coherent enough to make an intellectual point, seems to be arguing that meddling in history is a bad idea. Things  even awful things  happen for a reason. Me? I'm not sure I agree. Would that Hitler had been assassinated. But since this is all theoretical  I just checked my basement, and the only portals to the past available to me were my O-level certificates and a deflated Space Hopper  I find that I can't really care too much one way or the other.

People are commonly said to remember their location when told of President John F Kennedy's assassination, but many must also wish the place they had been on 22 November 1963 was Dallas, where they might somehow have diverted the motorcade or prevented Lee Harvey Oswald from entering the Texas School Book Depository. The possibility of such an intervention must number, along with its darker twin of going back and killing Hitler, among the principal fantasies of time travel, and is explored in the 54th work of fiction by Stephen King.

In 11.22.63, Jake Epping, a schoolteacher in Maine (a childhood reference point as recurrent in King's fiction as New Jersey in Philip Roth's), is summoned by the owner of Al's Diner, a local eaterie that has become popular but also suspect as a result of being able to sell, in 2011, burgers at near-1950s prices. The restaurateur, now mortally ill, has found a portal in his pantry that leads to a particular day in 1958, where the time-traveller can begin a stay lasting months or even potentially years, always returning two minutes later. Cancer has interrupted Al during a five-year mission to prevent the event that he believes to have misdirected American history: JFK's death. With the moral arm-lock of a dying man, Al passes on the task to Jake.

Time machines that travel backwards invite a writer towards period detail and nostalgia, and it is striking that King's device defaults to a year in which he would have been an 11-year-old schoolboy in Maine. Jake, who adopts the cover identity of real estate salesman George Amberson when he goes back, luxuriates in the unadulterated root beers and chocolate pies of an era before fast food.

"I wanted to see the USA in my Chevrolet," he sentimentally declares on the brink of one trip. "America was calling me." And, though the "temporal bedouin" from 2011 sometimes struggles with the lingo (what he calls a "motel" is a "Motor Court" there), the flashback America is largely a better one. Back in these days, baseball is played "as it was meant to be played" and Jake/George finds the prices astonishingly low except, interestingly, oranges and long-distance phone calls, both exotic luxuries at the time. Less heart-warmingly, a cancerous miasma of cigarette smoke clouds every 1958 scene and racism is standard.

The only sustained criticism of King, apart from the howls of some incurable literary snobs, has been his books' alternative use as weight-lifter's training aids and there are moments, early in this 700-page work, when we may wonder if the mission couldn't have begun in, say, 1962. But King has an advanced understanding of narrative structure and it's soon clear that his protagonist needs first to undertake a trial mission to establish the rules of intrusion. Running under the book is the question of whether we would have the moral right to dam the river of time, a dilemma explored through a fictional Hitler-like president in King's The Dead Zone (1979).

A novel about thwarting Lee Harvey Oswald is crucially different from one about killing Hitler because many readers will question whether the hero is going after the right man. Jake/George regularly frets that, even if he changes the shape of Oswald's day on 11.22.63, he may discover that the conspiracy theorists were right and JFK is taken out by another gunman from the grassy knoll or elsewhere.

This nagging doubt about the security of the history being altered is beautifully used by King, who also cleverly exploits a major fascination of time-travel or counter-history stories: the historical adjustments that result from meddling. While the latter parts of the novel deserve heavy protection against plot-spoiling, it can be said that the racist Governor George Wallace, Paul McCartney and Hillary Clinton are among those whose Wikipedia entries are intriguingly re-edited.

In a thoughtful afterword  in which King suggests that he partly intends the novel as a warning against "the consequences of political extremism" in contemporary America  the writer reveals that he first tried to write this book in 1972 but felt too close to the raw pain of the assassination. So this book makes, with the monumental Under the Dome (2009), the second recent case in which King has gone back in time to complete a project that previously eluded him.

With some senior writers, the dusting out of bottom drawers indicates creative stasis. But King, whose writing life represents among other things a model of canny career management, has waited until the right time for these novels. In these books, the reader feels the benefit of 40 years of narrative craftsmanship and reflection on his nation's history. Going backwards proves to be another step forward for the most remarkable storyteller in modern American literature.

Mark Lawson's Enough Is Enough is published by Picador.