The Enterprise travels back in time to Earth, 1968, to witness a historic nuclear crisis unfold. But once there, they encounter the mysterious Gary Seven (Robert Lansing) beaming in from another planet, and Kirk must decide whether his presence is a proper aspect of history or an alien threat. Meanwhile, Mr. Seven escapes his holding cell and begins conducting his undercover operation on Earth, centering on the scheduled launch of a nuclear device into orbit.
The time-travel motivation is dubious (why in the world would Starfleet risk timeline contamination to research history?), but the story has some good ideas. Unfortunately, the execution is off-kilter, with so much cross-cutting and off-pacing that the show turns choppy. Also, the episode comes across like the spin-off pilot show that it was intended as; at times it's more interested in providing a backdrop to a series that would never come to be than it is in making its story the priority.
Robert Lansing is on target as Mr. Seven, but Teri Garr is too annoying and unfunny as his secretary. The plot is reasonably good, but the bottom line is that I felt more like I was watching a good marketing ploy than I was watching good science fiction.