The Enterprise is drawn to a planet in order to synthesize a cure for a plague on another world and encounters a mysterious man and his winsome ward.
I really like this episode, despite some drawbacks. Flint is a sympathetic if misguided man who has suffered the curse of never being able to die. His solution, to create an android companion, is no where near as repellent as Kirk tends to believe. The mystery of Flint's identity is well-played, the scenes of Spock putting the story together are interesting. The secondary plot of working to distill a fever plague cure is workman-like but only meant to keep the main players on the planet. Of all the sturm and drang of this episode, WAY over-played by Shatner (though I like the ending scene of Spock helping him to forget), are Rayna's repeating of Flint's lines on loneliness, - "it's a hunger, a flower dying in the desert" - heartfelt and genuine. At the same time, it's probably best to just accept that Kirk could fall in love with a naive young android in 20 minutes time given that he's done something similar many times before. A ton of believability would be added by having the action take place over a few weeks rather than a couple of hours. Amazingly, Kirk is at least scripted as saying that he "put on a pretty poor show."
There are some real turkeys in Season 3, but I like this one, "The Empath", and "All Our Yesterdays" in going to the very heart of emotion and sacrifice and how they strike a universal chord about what it means to be alive. Since "Star Trek" was never really going to explore the idea of radically different life forms, at least these episodes scraped beneath the superficial aspects of our own species' behaviors.