Jake Epping is a recently-divorced
high school English teacher in
Lisbon Falls, Maine, earning extra money teaching a
GED class. Epping gives an assignment to his adult students, asking them to write about a day that changed their lives. One of the students, a learning-impaired janitor named Harry Dunning, submits an assignment describing the night his alcoholic father murdered his mother and siblings with a hammer and injured Harry, causing him brain damage; the story emotionally affects Jake, and the two become friends after Harry earns his GED.
Two years later, in June 2011, Jake stops by a local diner and speaks with the proprietor, Al, who asks Jake to meet him at his diner the next day. When Jake arrives, he is shocked to see that Al seems to have aged years since the previous day. Al explains that he is dying and that his appearance is attributable to his having time traveled and lived for years in the past. Al's method of time travel is a time
portal he discovered in his diner's pantry, which he used to transport himself to 1958. Doubting Al's story at first, Jake travels through the portal, where he encounters an addled wino whom Al has dubbed the "Yellow Card Man" due to the color of a card on the man's hat. Jake spends an hour in 1958 before returning to the present, after which Al explains that he's figured out the basics of how the portal functions:
- Every journey through the portal transports the traveler to September 9, 1958, at precisely 11:58 a.m.
- No matter how long someone stays in the past—hours, days, weeks, or years—only two minutes elapse in 2011.
- Past events can be changed; however, subsequent use of the portal "resets" the timeline and nullifies all changes made on the previous excursion.
- The "obdurate" past throws up obstacles to prevent history from being changed. Such resistance is proportional to the magnitude of the change.
Because the portal gives one the ability to alter the present by changing an event in the past, Al reveals that he concocted a plan to prevent
John F. Kennedy's assassination, hoping that doing so would change history for the better, as he attributes many bad things that happened in the world to events that would not have occurred had JFK lived. He spent four years in the past after entering the portal the previous night, traveling to
Dallas, Texas to track
Lee Harvey Oswald, plotting to kill the would-be assassin during his attempted murder of General
Edwin Walker. His delay was due to the fact that he wanted to be absolutely sure that Oswald was a killer and would act alone. Al developed cancer, so he had to give up his mission, knowing he would not live long enough to complete it. He recruits a reluctant Jake to complete it instead.
As an experiment, Jake travels back to 1958 to save Harry's family, who will be killed by his father, Frank Dunning, on
Halloween night. Despite many obstacles, he succeeds in saving all but one of Harry's siblings, then returns to 2011 hopeful he improved Harry's life...only to learn his actions led to Harry dying in Vietnam. While Jake is still trying to process this information, Al commits
suicide, forcing Jake to act immediately, before the death is known and the diner is sealed.
With no preparation, Jake re-enters the portal and discovers that the "Yellow Card Man" has cut his own throat and the yellow card is now black. He ignores it and kills Frank ahead of Frank's murderous rampage. After resolving one of Al's other missions—preventing a hunter from accidentally shooting a little girl—Jake makes his way, first to Florida, then to the small Dallas-area town of Jodie to wait for Oswald's arrival. Jake spends several years establishing his identity in the late 1950s, but comes to suspect history "harmonizes"—he keeps coming into contact with people of the same name, with similar events. He suspects saving one life may result in another person dying in their stead, for example.
He begins to stalk Oswald, renting apartments near the Oswalds' apartments. He begins to wonder if Oswald's only friend in Dallas,
George de Mohrenschildt, may somehow be involved in the assassination, and thus hesitates to kill Oswald ahead of time. He thinks de Mohrenschildt is a CIA resource who is supposed to keep an eye on Oswald, but may also be egging Oswald on to kill first General Walker, then JFK. Jake resolves to wait until the Walker attempted assassination before killing Oswald. However, he is unable to learn certain facts and is prevented from accessing several opportunities to kill Oswald.
Finally, the situation comes down to November 22, 1963. With everything going wrong in order to prevent him from his date with destiny, he is only able to reach Oswald seconds before the fateful moment when Kennedy's motorcade drives through
Dealey Plaza. Nevertheless, he successfully prevents Oswald from shooting JFK. In a rage, Oswald fires at Jake ultimately killing Jake's fiancée, Sadie, who came to help him. Oswald is then killed by outside fire. Jake spends a few dizzying days in Dallas, as investigations prove he was not Oswald's accomplice. He has conversations with both the President and the First Lady. He is numb to all of it, because he has already resolved to travel to 2011 and then back to 1958 again, in order to save Sadie. But as he leaves Dallas, he learns that there has been a massive earthquake in California, in which thousands have died. He realizes that it is a direct result of his actions.
When he gets to the portal, he finds that the degenerate Yellow Card Man has been replaced by Zack, a respectable looking man with a Green Card. The man explains that traveling through the portal does not
change the past, but rather creates new strings of time, stretching the bonds of reality. Guarding the portal is difficult, because the men dispatched to do so must keep myriad realities in their minds at all times. (The cards they wear function as a form of
radiation detector; a green card indicates the agent is healthy, but as they deteriorate, the card changes to yellow, then to red, and finally to black.) Eventually the process is so stressful that it drives most agents to mental illness or alcoholism, like the Yellow Card Man. Zack begs Jake to set things right again. Jake steps back through the portal, eager to see what a world without a Kennedy assassination has become. He discovers it's a nuclear winter-scarred landscape. 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 the nuclear nightmare the world is currently experiencing. Furthermore, there are frequent, massive earthquakes everywhere. Harry tells Jake a concise history of the world between 1963 and 2011, and it's not pretty. Jake quickly returns to 1958 and finds Zack much worse for wear. He tells Jake he must now go back to 2011 (since all of Jake's changes are now undone), and ensure 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. Looking up old records, he learns that Sadie survived an attack by her ex-husband, an attack she had only survived before by his rescuing her with the assistances of Deacon. In this timeline, Deacon had another of Sadie's friends with him to assist in subduing the attacker.
Jake goes back to Jodie, where Sadie is in her 80s and is being honored by the town with a celebration. The two share a dance.