For Dr. McCoy life takes two paradoxically divergent paths. In one he saves a woman from dying and in doing so alters Earth's history. In the other, he is prevented from saving the woman's life, allowing Earth's history to remain unchanged.
Spock, displaced in time, watches his closest friend heed his advice by allowing the love of his life to die in a traffic accident, thereby preserving Earth's history. Returning to the present, however, Spock confronts other such crises, and chooses instead to willfully alter the past. Challenged by the thorny demands of his logic, he will have to find a way to face his conflicting decisions.
James Kirk, displaced in time, allows the love of his life to die in a traffic accident, thereby preserving Earth's history. Returning to the present, he continues a storied career as a starship captain. Now, facing his own death, the very fabric of existence collapses across years and light-years, forcing him to race against and through time itself.