Professor Joseph Fitzgerald is teaching at Harvard.  One of his students asks to do a paper on the speech President Kennedy will be giving at the Trade Mart in Dallas, which places the timeframe of this story in November, 1963.  While in his office, Fitzgerald drops a coin from his pocket.  He picks it up, and we see it's a Kennedy half-dollar.  Since this is before JFK is killed, we now know that something odd is going on.  A buzzing sound fills the air, and he closes the curtains. A woman appears in his office, where they discuss what they do. Fitzgerald is a time travelling historian, who is doing his research in this time period.  He's really from a time period about 209 years into the future. She tells him he shouldn't have the half-dollar, which is strictly against protocols since it's from the year after the year he's in. She also tells him he doesn't have to go to Dallas, but he defends his research by saying these are arguably some of the most turbulent times in the history of the US.  He feels that he never really got to know JFK, even though he met him at a political function, and now he's got to watch him die. She leaves him with a Chinese blessing, disappearing after she pecks at an unusual watch on her wrist. 

Fitzgerald timejumps to Dealy Plaza to watch the motorcade, startling a bystander when he suddenly appears beside him.  Fitzgerald begins filming the motorcade with his holographic camera. He pans up and sees the rifle in the window of the book depository. Caught up in the event, Fitzgerald runs down to the hill and warns the President when he sees the gun in the Book Repository window.  The motorcade speeds off, with the President unhurt and Fitzgerald pushed down by a Secret Service agent.  He's put in a car and taken to the airport. When they arrive, Vice President Johnson mentions that he can't go back to Washington with the President.  Several large tornadoes have suddenly shown up in Texas, and he has to help with damage assessment.  Fitzgerald looks uncomfortable when he hears this news. 

Since he's saved the President, he's invited to accompany JFK to the White House.  He and the President talk about the attempted assassination, and when the President hears Fitzgerald's name, he asks if they're related; Fitzgerald says no.  Fitzgerald is flipping his Kennedy half-dollar on the plane, and it rolls under the seat. When they get to the White House, Fitzgerald rememberss that he left his camera, and his Kennedy half-dollar, on Air Force One.  The President's principal secret service agent, Ray, promises to get the camera for him. 

Fitzgerald is staying at the White House, and he is consulting his time travel device, which is the unusual watch he wears. Because he's created a time rift through his actions, he finds out that the timeline he's now created is completely unviable and the world will end in war and famine because of it. He's told that to fix this rift, Kennedy will have to die in Dallas, as he originally did.

When Ray finds the half-dollar by the camera, he takes it to have it looked at.  The Treasury man says it's not a fake; it's a mint grade coin, but they never put a live President on a coin.  Ray is suspicious now.   

Ray takes the coin to the President, who isn't worried.  He thinks the coin is a fake.  But Ray says it's not; no one can fake a coin that good.  They call Fitzgerald in and ask him about the coin.   They are skeptical, and the President is getting angry, but Fitzgerald tells him it's been in his family for over 200 years. The President is incredulous, but is eventually convinced when Fitzgerald turns on his camera and shows the President a hologram of the motorcade from Dealey Plaza.  Technology of this caliber is unknown in 1963.  He tells the President that he was sent back to study the Kennedy administration. Fitzgerald then tells the President they are related.  While they talk, the President figures out that he shouldn't have survived the assassination attempt today, and bravely tells Fitzgerald to send him back so time can be repaired.  Fitzgerald hesitates, then removes his time ring and gives it to the President, who puts it on and vanishes, assuming he's going back to the motorcade. 

However, when Ray is startled by the President leaving, Fitzgerald says that if he wants to protect his President, he has some work to do.  Fitzgerald goes back in time and takes the President's place, and he is the one that is killed.  Ray is in the hospital ER with Fitzgerald's body, where we see Fitzgerald's friend, who is studying in this period as well. She tells Ray that her people study all the different timelines. and she knew the last time she saw Fitzgerald that she would never see him again. She says that the Chinese phrase she said to him as she left him was "Goodbye, old friend, goodbye." It was all she could do.

In the end shot, we see a very much alive John F. Kennedy wearing a futuristic suit, 200 years in the future, giving a speech to a group of students about humanity.