A person travels back in time and changes history somehow.

Examples:
In tz1985e1x20a "Profile in Silver", we saw history professor Dr. Joseph Fitzgerald travel back in time from the year 2172 to quietly observe President John F. Kennedy. Alas, when the sinister moment arrived Joseph could not stop himself but impulsively intervened to save the much admired President. This action had unexpected and dire consequences for the world.