Point One: Stephen King is often dismissed as a mediocre wordsmithing machine, cranking out modest midlevel fare, a sort of literary Bon Jovi. Sure, some of his stuff isn't great, but when you're that prolific, there will be clunkers. I've never been partial to his horror stuff only because that's not a genre I relish, but some of his more mainstream stuff is really quite splendid. Read his original novellas Apt Pupil, The Body, or Rita Hayworth and the Shawshank Redemption, for instance. Under the Dome is another example of his ability to sustain excellence in long form. Perhaps my favorite of his is his time-travel opus, 11/22/63.
Point Two: Time travel has long been a literary device, dating back to Twain's 1889 Connecticut Yankee and Wells's 1895 Time Machine. Since then we've seen the theme take center stage in Back to the Future, Terminator, Star Trek, Bill and Ted's Excellent Adventure, Harry Potter, and many more. This year's all-time cinema champion Avengers:Endgame exploited the trope effectively, but also wrestled with the central tenets of the premise. When you mess with time, it tends to mess back, right? That concise summation is the central conceit of King's 11/22/63. Changing the past is difficult because it doesn't want to be changed. And if you do change it, all you do is create another timeline and warp the fabric of the multiverse. Meddler.
The protagonist of the book is Jake Epping, a fairly run-of-the-mill King character, living in Maine and teaching and leading a fairly pedestrian existence. When he stumbles upon a time portal in a diner, he embarks on an adventure that becomes a mission. He's recruited by the portal's previous user to help prevent the JFK assassination and right the wrongs that event caused, from the Vietnam quagmire to domestic division. This being King, there's no straight line from here to there. Epping spends a lot of time wandering around in the late 50s and early 60s, having some romance and meeting some weird time cops and fighting dudes with hammers in backyards. Yeah, it's still Stephen King. Time itself, reluctant to be tampered with, throws up obstacles but Jake persists, and eventually manages to save Kennedy on November 11th. Except it didn't fix things - it made them worse. Meddler.
The conclusion to the tale is sad and frustrating - essentially, time is immutable, and there's nothing you can do about what has been. That's liberating, too, because we're free to focus on tomorrow, not yesterday.