Little Anthony Fremont controls an entire town with his ability to read minds and make people do as he wishes. Which is a real good thing.
Kirk and three of his officers are accidentally transported into a parallel "mirror" universe where violence, greed, and evil are commonplace.
While seeking a cure for a fever ravaging the Enterprise, Kirk and Spock encounter Flint, a hermit-like Earthman, and his beautiful young ward.
When the Enterprise brings aboard Klingon prisoners, an alien entity pits both sides against each other in an ever-escalating struggle.
The Enterprise's command crew must thwart an invasion by aliens from another galaxy who plan to conquer this one.