Star Trek: The Original Series
The City on the Edge of Forever
S1 E28:
Accidentally overdosing on medication, a psychotic McCoy leaps through a time portal into the 1930s and somehow disrupts Earth's future so that the Federation and Starfleet do not exist. The stranded Kirk and Spock follow him back to set things right, where a tragic love tests Kirk's resolve.
Starring
Joan Collins, John Harmon, Hal Fieberling