Guest stars:

Diana Muldaur           Dr.Ann Mulhall, astrobiology

Sargon, ancient life evolved into    pure energy (William Shatner)
Thalessa, wife of Sargon (Diana Muldaur)
Hanock, from 'the other side'(Leonard Nimoy)

Crew Appearances:
Kirk, Spock, Mccoy
Uhura
Sulu
Scott
Nurse Chapel
2 security guards who didnt beam down
 
Three aliens who have been without bodies for centuries - having lived in a purely mental state - ask Kirk, Spock and McCoy to "loan" them their bodies so that the trio can construct androids that will house their minds. After some reluctance they agree to do so, although the being that takes over Spock decides that he likes being flesh and blood, and he will stop at nothing to keep his host, even at the expense of his two companions or the crew of the Enterprise.

Unnamed planet: class M, close to Earthlike, much older, but dead for the past 1.5 million years when its atmosphere was 'ripped' away by a cataclysm. It is located 100's of light years past where any other earth ship has explored.

A chamber, hidden under 100 miles of solid rock, with a breathable atmosphere

metabolic reduction injection: drug that Hanock develops to reduce the heart action and bodily functions to normal. Hanock prepares a different formula for Sargon to kill him and kirk.

Megaton Hydrocoils: in the form of a 'drop of jelly' used in place of microgears and pullies for android bodies.

Quotes:
"Even a Vulcan can't know the unknown" Spock.
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"Sealed inside this recepticle is the essence of my mind." Sargon.
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"You're going to WHAT!? Are they alright in the head?" Scotty.
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"He's dead." McCoy about Kirk.
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"I wil not peddle flesh. I'm a physician." McCoy.