Star Trek: The Original Series: Crucible: McCoy: Provenance of Shadows
For Doctor Leonard McCoy, life takes two paradoxically divergent paths. In one, displaced in time, he saves a woman from dying in a traffice accident, and in doing so alters Earth's history. Stranded in the past, he struggles to find a way back to his own century. But living an existence he was not meant to, he will eventually have to move on, and ultimately face the shadows born of his lost life.
In the other, he is prevented from saving the woman's life, allowing Earth's history to remain unchanged. Returning to the present, he is nonetheless haunted by the echoes of an existence he never lived, and by fears which will bring him full circle to the shadows he never faced.