Return to Tomorrow

Telepathic aliens take control of Kirk and Spock's bodies with the intention to build new, mechanized bodies for themselves. After receiving a distress signal from a planet deep in the galaxy and far from the last charted area, the Enterprise is contacted by a life form of pure energy that wishes Kirk, Spock, Dr. Mulhall and Bones to beam down. They meet Sargon, a conscious mind trapped in a machine. Sargon explains that their civilization travelled space just like Kirk centuries ago and left people in various star systems to colonize. But this planet suffered a war where all but a few people destroyed themselves. Sargon, his wife and another remain alive like this and wish to take control of Kirk, Spock and Dr. Mulhall's bodies to make android bodies for themselves. The Enterprise accepts their offer after deliberation and Sargon begins work.

Summary from Wikipedia, CC BY-SA 4.0.