Plot
In 2011, Jake Epping, a divorced high school English teacher in Lisbon Falls, Maine, is summoned by friend Al Templeton, the owner of a local diner. He finds Al is ill with terminal lung cancer, despite appearing perfectly healthy the night before. Al instructs Jake to step into the back of the diner's pantry, where Jake finds a time slip leading to Lisbon Falls as it existed on September 9, 1958. After exploring the town, Jake returns to 2011 and learns that the portal leads to the same moment of the same day every time it is used, and that a visitor will always return to the present by a margin of two minutes.
Because the portal gives one the ability to alter the present by changing an event in the past, Al reveals that he had concocted a plan to prevent John F. Kennedy's assassination, hoping that doing so would stop the Vietnam War and change history for the better. He spent four years in the past after entering the portal the previous night, travelling to Dallas, Texas, to track Lee Harvey Oswald, plotting to kill him during his attempted murder of General Edwin Walker. However, due to his cancer, Al is unable to continue his mission. He recruits a reluctant Jake to complete it.
As an experiment, Jake travels back to 1958 to save the family of learning disabled janitor Harry Dunning from being attacked by their father on Halloween. Using the alias "George Amberson", Jake buys a car and travels to Harry's hometown of Derry, Maine; he finds Harry's father, Frank, and tracks his movements. After saving all but one of Harry's siblings from Frank's brutal assault, Jake returns to 2011, to find that he indirectly caused Harry to die in Vietnam. Al dies by suicide.
Resigned, Jake re-enters the portal, travels back to Derry, and kills Frank ahead of his rampage. Jake eventually settles in Jodie, Texas where he becomes a full-time English teacher at a local consolidated school. He spends several years establishing his identity. Jake winds up falling for school librarian Sadie and having a positive effect on the students of a local high school. Jake reveals to Sadie that he is from the future and his plans to prevent Kennedy's assassination. Sadie is saved by Jake from her ex-husband, John. Ultimately, the pair rush to the Texas School Book Depository, where Jake interrupts Oswald. This saves Kennedy; however, Sadie is killed.
Jake returns to a dystopian, war-torn future brought about by his actions. He finds that after Kennedy served two terms, segregationist George Wallace was elected president in 1968. The Vietnam War not only still occurred, but escalated into World War III, the Civil Rights Act of 1964 was never passed, Martin Luther King Jr. was assassinated by the FBI, and the US is now fractured. Earth is also plagued by constant, devastating natural disasters, and the sky itself appears to be tearing. Jake discovers that each trip to the past causes a separate reality to be formed, and that his journey has caused time to begin falling apart. He realizes that he cannot save Kennedy for the good of the future, prompting him to return to the past to "reset" the results of his intervention. Jake finds that Sadie survived without his interference, and is being celebrated as a "Citizen of the Century" in Jodie, and he goes to the party being held.