Jake Epping is a thirty-five-year-old high school English teacher in Lisbon Falls, Maine, who makes extra money teaching adults in the GED program. He receives an essay from one of the students – a gruesome, harrowing first person story about the night 50 years ago when Harry Dunning’s father came home and killed his mother, his sister, and his brothers with a hammer. Harry escaped with a smashed leg, as evidenced by his crooked walk.
Not much later, Jake’s friend Al, who runs the local diner, divulges a secret: his storeroom is a portal to 1958. He enlists Jake on an insane – and insanely possible – mission to try to prevent the Kennedy assassination. So begins Jake’s new life as George Amberson and his new world of Elvis and JFK, of big American cars and sock hops.
As an experiment, Jake travels back to 1958 to save Harry's family, who will be killed by his father on Halloween night. Using the alias "George Amberson", Jake buys a car and travels to Harry's hometown of Derry, Maine. Jake is able to find Harry's father, Frank Dunning, and track his movements. After saving all but one of Harry's siblings from Frank's brutal assault, Jake returns to 2011 hopeful that he improved Harry's life—only to learn that his actions indirectly led to Harry being killed in the Vietnam War. While Jake is still trying to process this information, Al commits suicide. Jake is forced to act immediately before Al's death is known and the diner is demolished. When Jake re-enters the portal, he discovers that the "Yellow Card Man" has killed himself; the yellow card is now black.
Jake travels back to Derry, where he kills Frank ahead of his rampage. Jake then makes his way to Texas to wait for Oswald's arrival; he settles in Jodie, a small town located near Dallas. There, he is hired as a full-time English teacher for a local consolidated school, quickly becoming popular with the students and faculty. He also starts a relationship with Sadie Dunhill, the school's librarian. However, the courtship ends when Sadie becomes suspicious of Jake's anachronistic speech and his singing of "Honky Tonk Women" by the Rolling Stones — a song that does not yet exist. Not long afterwards, Jake quits his job and leaves Jodie, renting a Fort Worth apartment across the street from Oswald's future residence. He monitors Oswald's activities with audio bugs and an omnidirectional microphone.
Through the bugs, Jake witnesses Oswald's abuse of his Russian wife and his conflicts with his overbearing mother. Around this time, Jake reconnects with Sadie and reveals that he is from the future, proving his claims by correctly predicting the outcome of the Cuban Missile Crisis; he eventually reveals his full plan of preventing Kennedy's assassination. Sadie is reluctant to believe Jake at first, but her love for him leads her to support his efforts. Meanwhile, Jake becomes hesitant to kill Oswald when he sees his friend, George de Mohrenschildt, seemingly egg on the would-be assassin to kill Walker and Kennedy. Jake is unable to interfere in the Walker attempt when Sadie is disfigured by her psychotic ex-husband. Jake himself is beaten by a bookie who lost money due to Jake's knowledge of future sporting outcomes. Jake spends three months recovering from the beating and the subsequent memory loss.
Finally, the situation comes down to November 22, 1963. With everything going wrong in order to prevent him from stopping the assassination, Jake is only able to reach Oswald's sniper's nest at the Texas School Book Depository moments before Kennedy's motorcade drives past. Nevertheless, he successfully prevents Oswald from shooting JFK. In a rage, Oswald fires at Jake, who falls down due to his leg injury. The shot misses Jake and hits Sadie. The noise of their confrontation draws the attention of the United States Secret Service and police, who fire through the window from the outside, killing Oswald. Sadie dies in Jake's arms. Jake immediately becomes a national hero, being personally thanked by President Kennedy and his wife. The FBI suggests that Jake disappear for a time until the situation dies down. Agonized over Sadie's death, Jake resolves to return to 2011 and back to 1958 in order to repeat his journey in order to save both Kennedy and Sadie.
When Jake arrives back in Derry on the morning of Nov. 26, he learns there was a massive earthquake in California the day before in which thousands died. He suspects that it is a direct result of his actions.
Returning to the portal, he finds that the Yellow Card Man has been replaced by a respectable looking man with a Green Card. He reveals himself to be a sort of time "guardian" who explains that many other time portals exist in the universe. The time portals, the guardian explains, are temporary "bubbles" in time, which will disappear as the physical environment they reside in changes. The man explains that traveling through the portal does not erase the past, it creates a new string until the portal is destroyed. The larger the change, the more unstable reality becomes - even minor changes will eventually overwhelm reality. The yellow/green card is shown to be a type of film badge dosimeter that measures mental degradation caused by exposure to time line changes rather than radiation; green resembles healthy, yellow is fair, and black is unstable. Guarding the portal is difficult since he has to keep the myriad realities in his mind at all times. Eventually this drives the guardian to mental illness and sometimes alcoholism, like the Yellow Card Man. He begs Jake to set things right again, otherwise reality itself could possibly be destroyed.
Jake steps back through the portal, eager to see what the world has become like. He discovers that Lisbon Falls (and the world) is now a lawless dystopia. He meets a familiar looking man, who turns out to be Harry Dunning, whose life he saved long ago. Not a brain-damaged janitor in this incarnation, he is a wheelchair-bound survivor of this time string. Harry tells Jake a concise history of the world between 1963 and 2011, involving nuclear weapons, environmental collapse from fallout and the greenhouse effect, energy shortages, domestic terrorism, general lawlessness and continued unexplained earthquakes, all of which are slowly destroying the planet. Jake quickly returns to 1958, and finds the Green Card Man much worse for wear. He tells Jake he must now go back to 2011, and see that the portal is closed. Instead, Jake goes to a hotel and contemplates returning to Texas, to Sadie. Ultimately, he returns to his own time, having changed nothing, and history is restored to its original track after the diner is demolished. Learning that Sadie survived the confrontation with her ex-husband without his interference, he goes back to Jodie, where Sadie is now an old woman. The two lovers from different timelines share a dance.
'Then the music takes us, the music rolls away the years, and we dance."
The novel had been optioned by Jonathan Demme to be made into a film. As of 6 December 2012, Jonathan Demme has left the project, due to disagreements with King over what should be involved in the script.