High school teacher Jake Epping is approached by his friend, Al, who has access to a portal which transports whoever uses it to the year 1960, where Al suggests Jake can avert the assassination of John F. Kennedy.

In an attempt to commit one act which could make a difference, Jake remains in Holden, where he tries to prevent Harry Dunning's father from doing the unthinkable. However, the past has other ideas.

Jake strikes up an unlikely friendship with Bill, and the pair travel to Dallas, where Jake begins living a double life, and both of them trail Lee Harvey Oswald.

Jake finds his credentials questioned, before he and Bill intercept a conversation between George and Oswald, which leads to unexpected trouble. Meanwhile, Sadie's husband unexpectedly resurfaces.

As questions mount about Jake, his double life begins to unravel, while the past interferes with the plans he and Bill have for Oswald.

As the assassination date approaches, Jake is horrified to learn how involved Bill has become with Oswald and must take drastic action, while the past continues to conspire against Jake.

Unable to recall his mission, Jake turns to Bill for help, with unexpected results, while Sadie attempts to refresh Jake's memory.

The past makes every effort to prevent Jake from altering the events of November 22, 1963, and saving the President, with his own life hanging in the balance.