Jake Epping is a high school English teacher from small-town Lisbon Falls, Maine. At the start of the novel, he reveals that he is going through a bitter divorce with his soon-to-be ex-wife. The two simply drifted apart and no longer were in longer.
In an effort to get his mind off his divorce, he goes to Al's diner, a spot for locals to gather and chat. While going through the divorce, the two talk about President John Kennedy and his assassination, but the subject quickly moves out of Jake's mind. Several years later, his divorce now finalized, Jake returns once again to Al's. Now, Al reveals that he has terminal cancer, something that developed overnight. The two revisit the topic of President Kennedy, and Al reveals to Jake that he has a portal that is capable of transporting users back to September 9, 1958. Al had long wanted to use the portal to travel back in time and stop President Kennedy from being assassinated to make the world a better place and to stop the Vietnam War from escalating. Now, with his condition, he is no longer able to do that, so he asks Jake for his help. After some trepidation, Jake agrees and travels back to 1958, where he'll have just over five years to prepare to stop President Kennedy's assassination.
Just before entering the portal, Jake adopts an alias for when he was in the 1950s and '60s. The name he took for the alias was George Amberson. While in that time period, Jake meticulously planned and researched so that he could successfully intervene in the looming assassination. However, his five-year journey will not be a straightforward one. For one, he starts a romantic relationship with a woman named Sadie, a librarian whom he falls deeply in love with. The two become exceptionally close very quickly, and Sadie helps Jake in his mission. After his divorce, Jake never stopped searching for a companion. Until Sadie, he hadn't found the right person.
To prepare for his mission, Jake begins to investigate Lee Harvey Oswald, the person who he thinks will kill Kennedy. However, Jake struggles to determine if Oswald is acting alone or if he is acting as a part of a larger conspiracy. Through his surveillance of Oswald, Jake learns that his life is difficult, and he begins to develop a modicum of sympathy for Oswald.
As Jake gets closer to the date of the assassination, he realizes that his path forward will be difficult. There are forces beyond himself preventing the past from being changed. For example, Jake's memory is wiped in the weeks leading up to the assassination attempt, nearly causing him to forget what he came to the area for.
The climax of the novel is on November 22, 1963, the date of Kennedy's assassination. Jake and Oswald have a dramatic confrontation at the Texas School Book Depository, where Oswald plans to shoot Kennedy. That confrontation ultimately leads to Oswald's death, thereby saving Kennedy and earning the respect of Mr. and Mrs. Kennedy and the nation. But his actions force him to give up the love of his life, Sadie.
Upon returning to 2011, Jake discovers that his actions have destroyed the world. Nuclear bombs were dropped across the United States and Russia, killing much of the population and causing seemingly constant natural disasters. Realizing his grave mistake, Jake decides to reset the timeline by going back and allowing the assassination to proceed as it originally did.
The story concludes with Jake visiting a present-day Sadie, now an elderly woman. When Sadie asks who she is, Jake tells her, "Someone you knew in another life, honey."
