In this Channel 73 video, Eddie discusses Assignment: Earth, the Star Trek spin-off that never happened. You might remember "Assignment: Earth" as the March 29, 1968 final episode of season 2 (starring Robert Lansing and Terri Garr). After Gene Roddenberry proposed the Star Trek Assignment: Earth spin-off to NBC and they rejected it, he re-rolled it into a Trek episode to function as a pilot (and convenient segue) for the new series. The "pilot" was rejected by the network as well and that was the end of that.
Here's the full Assignment: Earth episode.
Channel 73 even posted a fan-made intro for the series that never was: