Return to Tomorrow

Enterprise responds to odd readings, which lead the crew to a planet similar to Earth, except that it is much older, and a cataclysm stripped its atmosphere away a half million years ago; it has been dead ever since. But from somewhere on that world comes a voice. Something has evidently endured 500,000 years – the last survivors of the civilization that flourished here.

Synopsis adapted from TVmaze (CC BY-SA 4.0)