From a planet devoid of life for half a million years, the Enterprise hears the voice of a powerful being (Sargon- voiced by James Doohan), who can control ship. He transports the landing party (Kirk, Spock, McCoy and astro-biologist Ann Mulhall) to room miles underneath the planet. However, the security guards they planned to take along were prevented from coming along. Sargon is one of only three survivors of an intelligent race made of pure energy/pure thought. They started life on Earth and spread out to many other places, so refer to our heroes as “my children.”
Suddenly, Sargon possesses Kirk’s body! He requires Spock’s and Dr. Mulhall’s bodies, too, but promises they will be returned after they build advanced robots to house themselves. Sargon gives them time to freely make up their minds and beams them back to the ship. McCoy warns against hosting the lifeforms, but Kirk, Spock, and Dr. Mulhall (Diana Muldaur- Dr. Pulaski on ST:TNG) are intrigued. We soon see that the aliens can only survive in human bodies for short periods of time w/o killing their surrogates!
It must’ve been fun for Shatner and Nimoy to take on two roles as aliens inhabiting the bodies of Kirk and Spock. Shatner as Sargon goes from over-the-top dramatics to being dignified and caring. His scenes w/ his wife Thalassa (in the body of Dr. Mulhall) are sweet and touching. Nimoy gets to play Henoch, who seeks to remove Sargon from the equation and take over. Nimoy has an excuse to play w/ different emotions; he creates a twisted (yet smooth-talking and smiling) villain. Henoch tries to trick Nurse Christine Chapel, who we know loves Spock.
This ep and its writer, John T. Dugan, earned a WGA Award nom in the category Best Written Dramatic Episode in 1968. Dugan wrote the original script after he had read an article about highly sophisticated robots. In his original draft, Sargon and Thalassa continue their existence as spirits w/o bodies, floating around the universe. However, Roddenberry (as he often did) re-wrot the script to change the ending (w/ the aliens fading out into oblivion). This is the reason Dugan put his pen name (John Kingsbridge) in the credits.
 Dr. McCoy’s objections are warranted and well expressed, but a final centerpiece speech by Kirk explains the risks and rewards of flight, space flight, sciences and alien encounter. He states “Risk is our business” in a well written and delivered plea.
 It’s haunting, tragic, deeply romantic, dream like, and sensual. The idea that god like aliens long to be human, to have all the feelings and emotions we take for granted, is deeply inspiring.
I loved how the “temptation” of Thalassa was so Biblical, with the suave Henoch in the role of the serpent.
-Excerpts from IMDB reviews