A small-town English teacher, Jake Epping, learns from a friend that there is a portal that leads from the present back to 1960. Jake's friend asks him to help him stop John F. Kennedy's assassination by going back in time and finding out who really killed Kennedy... and killing them.
Rather than return to the present, Jake travels to Kentucky to try and save Harry and his family. However, the teacher soon learns that the past pushes back.
Jake and his new ally go to Dallas to find out if Lee acted alone in killing Kennedy. However, Jake runs into romantic entanglements while posing as a substitute teacher in 1962.
Jake finds out about Sadie's past with Johnny, and she worries that she's ruined their relationship. Meanwhile, Bill draws closer to Marina, and George tries to introduce Lee to his friends.
Johnny comes after Sadie, preparing to kill her and Jake... just as Jake has to determine if Lee plans to shoot Walker.
Jake has a falling-out with Bill after discovering Bill's involvement with Marina. Meanwhile, the teacher learns that Lee acted alone, but time intervenes when Jake prepares to kill the man.
After his beating, Jake struggles to remember why he came to 1963 and what will happen to JFK. Meanwhile, the day of the assassination draws closer and Lee prepares for a fateful encounter.
Jake races against time--literally--to stop Lee Harvey Oswald and save JFK. However, when he returns to his own time, Jake discovers that things didn't change as he expected.