The Enterprise's mission leads them to uncharted territory, hundreds of light years beyond the territories explored by any Earth ship, pursuing a mysterious signal that turns out to originate from a world dead half a million years, sent by an alien that calls itself "Sargon" and begs the Enterprise for assistance.
It turns out that Sargon is one of only three aliens who survived the cataclysmic war that destroyed the planet millennia ago, and only at the cost of secluding their mental energies inside of orbs and forsaking their physical forms. Sargon begs with the Enterprise to allow the three of them to temporarily borrow the bodies of three of the Enterprise's crew, that they can have bodies long enough to employ their hyper-advanced technology and create robot bodies to inhabit instead.
After much deliberation, Kirk, Spock and Dr. Mulhall
agree to become the hosts for the three aliens. Unfortunately, Henoch, the alien who chooses Spock's body, decides he would rather keep an existence of stolen flesh and blood, and makes plans to sabotage Sargon's plan by indirectly killing Sargon and persuading Sargon's wife, Thalassa, to side with him.
- Agony Beam: Thalassa and Henoch both use their mental abilities to inflict pain. Thalassa grows a conscience and stops. Henoch, on the other hand....
- Atmosphere Abuse: The Enterprise finds a planet whose atmosphere was ripped away by a cataclysm half a million years earlier.
- Cessation of Existence: Sargon says "Thalassa and I must now also depart into oblivion" before he dies.
- Dare to Be Badass: "I'm in command. I could order this. But I'm not... because... Dr. McCoy is right in pointing out the enormous danger potential in any contact with life and intelligence as fantastically advanced as this. But I must point out that the possibilities, the potential for knowledge and advancement is equally great. Risk... risk is our business! That's what this starship is all about... that's why we're aboard her!"
- Possibly the franchise's greatest monologue.
- Energy Beings: Sargon, Thalassa and Henoch qualify.
- Foreshadowing: Henoch's comment about wondering that the Vulcans had not conquered Earth could be an early sign that he's the bad guy.
- Getting Crap Past the Radar: Nurse Chapel is quite pleased about having had Spock inside her.
- Gilligan Cut: "You're going to WHAT?"
- Grand Theft Me: Sargon and Thalassa only intend to do this temporarily, long enough to construct robot bodies to hold them instead. Henoch has more permanent designs on Spock's body.
- Green-Eyed Monster: Henoch has more than a little of this.
- Hey, It's That Guy!: Dr. Ann Mulhall resembles a younger, less of a Dr. Jerk Dr. Pulaski. It's quite possible that she also has an identical cousin named Miranda Jones who wears a sparkly sonar shawl and carries her lover in a briefcase.
- Kissing Under the Influence: Sargon and Thalassa smooch no less than 3 times while borrowing Kirk and Mulhall's bodies.
- Large Ham: When Sargon is in Kirk's body, the hamminess is Up to Eleven.
- "Have no fear, Sargon's here!" (from the blooper reel)
- Nimoy is clearly having a ball getting to play someone completely different from Spock.
- Our Ghosts Are Different: The G word is never used, but this is essentially what Sargon, Thalassa and Henoch are.
- Power Echoes: When the aliens possess humans they gain echoing voices.
- Psychotic Smirk: Henoch gives a few in Spock's body.
- Red Shirts: Two of them are ready to beam down with the landing party, and no doubt breathed a sigh of relief when they didn't beam down after all.
- Sense Freak: All of the aliens enjoy the sensations of life again after taking human bodies, but none allow it to affect their work, though Henoch does use the fact that their robot bodies will not have this sensory ability to try and persuade Thalassa to side with him.
- Soul Jar: The spheres that contain Sargon, Thalassa and Henoch function as these. Later, Sargon uses Nurse Chapel as one for Spock's mind.
- Sufficiently Advanced Alien: Sargon, Thalassa and Henoch, aliens who survived the destruction of his world half a million years ago by becoming Energy Beings of pure thought.
- Telepathy: Sargon, Thalassa and Henoch are capable of this. Henoch abuses the power to brain wipe Chapel.