Assignment: Earth
Orbiting 1960s Earth on an observation mission, the Enterprise intercepts Gary Seven, a fellow time traveller claiming to have an important mission there. He'll act with or without their help.
Clearly this was intended as a spin-off pilot. Assignment: Earth, The Series didn't work out, but it did allow for a unique episode in that it's shot from Gary's perspective, not that of our Starfleet officers. This technique could have been used much more extensively - how does the energy being see? What are the Klingons thinking? - but wasn't. The branch of time travel theory that says everything will happen as it should, even when altering the past, is employed. Apart from the enigmatic and charming Mr. Seven, the new characters aren't too appealing. Teri Garr's secretary isn't funny, and little is known of the shape-shifting cat lady. (The second this season: consider Slyvia in Catspaw.) Kirk finally preaches directly on the dangers of nukes, and there is a sense that the fate of Earth rests with these men. Kirk might be reluctant to let someone else save the day, but this is the only time it happens. Good show.
Spock Saves the Day: Spock helps discern Seven's true motives . . .
Oops: . . . but not before threatening his mission.
Developments: With even Kirk a side character in this story, his first officer doesn't get much time to shine either.