This was a pilot episode of a new series produced by Gene Roddenberry about a time traveler named 'Gary Seven'. And the idea was introduced by having the stars of this potential series appear on a Star Trek episode. Robert Lansing appears uncredited as this main character (why he wasn't listed in the credits is odd considering he was obviously the star). I liked Lansing's cold and somewhat rigid style--I really thought it worked for his character, but I also realize this might have also left some feeling rather cold about him. He wasn't exactly 'Mr. Personality' in the episode. Instead, the major emoting was done by his ditsy assistant played by Teri Garr. It seems the spin off series was to have been about a man who traveled from a far more advanced world to secretly help mankind.
However, when Mr. Seven accidentally lands on the Enterprise, they are shocked to see him and think he's up to no good. You see, the Enterprise actually meant to go back to the 20th century to do some unobserved data collection for historical reasons and there is no conceivable reason why they should intercept a space traveler all the way back then. How all this is resolved is something you'll have to see for yourself.
As for the strengths of the show, the acting was very good and humor was infused now and again to make the show seem not so heavy-handed--which I appreciated. Unfortunately, the pilot was never picked up--and Roddenberry had a lousy track record in trying to bring other sci-fi shows to television (such as his two attempts with the character Dylan Hunt). And considering that it was pretty entertaining and a very interesting concept, I always feel a bit wistful when I see this episode.