The Enterprise is hurled back through time to Earth of the 1960s, where they inadvertently become the subject of Air Force Captain John Christopher's (Roger Perry) UFO sighting. After beaming him aboard the ship following an accident that destroys his jet, Kirk is forced to prohibit him from returning to Earth and contaminating the timeline with knowledge from the future. The only problem: Removing Christopher from Earth would also contaminate the timeline, because his unborn son would not be able to make a vital contribution to Earth's history as the future knows it.
As Trek's first time-travel outing, "Tomorrow Is Yesterday" is a brilliantly fascinating story, beginning with its exciting opening shots of the Enterprise flying through Earth's sky, and continuing through an adventure where the crew must remove all traces of their presence in the past by breaking into an Air Force base and stealing video recordings of the Enterprise. All of this is a great deal of fun, but the subtext of "Tomorrow" is what really stands out here: Through Christopher, this becomes a story analyzing the significance of any random individual and how they can make a difference in the world. And putting Christopher in the center of the story allows us to see the Enterprise and the future through his eyes.
True, the ultimate solution to the story's problem has its share of inconsistencies and loopholes (why is it traveling back in time would only clear Christopher's memory and not the Enterprise crew's?), but who really cares? This was a pioneer time-travel outing for Trek, and a great one at that, beginning a tradition of storytelling open to limitless possibilities.