this future historian who was distantly related to Kennedy comes back in time to study the pre-assassination days. Except while he is watching the parade about ten seconds before the shot he suddenly rushes the car and shouts "Mr. President, get down, get down!" And Kennedy is unhurt, even as the shots ring out.
But then quickly these huge tornadoes spring up and there is news that Khrushchev was assassinated.
It turns out that saving Kennedy has warped the space time continuum in unexpected and bad ways.
So the guy has to go back in time again and allow the assassination of Kennedy to happen.
Only he decided to take the place of JFK and the episode closes with JFK giving a lecture in the future, lauding the heroism of that future agent and saying "this too, is human".