Original air date: February 9, 1968
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.
Return to Tropes:
- Absentee Actor: Chekov doesn't appear in this episode.
- 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, implying a disbelief in any sort of life after death. The idea is muddled a bit by his suggestion that him and Thalassa will be together forever after their deaths, which sort of requires them to still exist after death. The inconsistency is a result of the show's producer adding in the "oblivion" part to fit with his atheistic philosophy, while the rest of the final speech was penned by the Catholic writer of the rest of this episode. He had written "into eternity" — they would still exist, just as disembodied spirits — and Roddenberry changed it.
- Cradling Your Kill: In the James Blish novelization of the episode, Kirk, nearly breaking down in tears, does this with Spock's 'dead' body after Henoch has been forced to flee. Kirk didn't give the lethal injection himself, but it was done on his orders.
- Dare to Be Badass: Some would call this the franchise's greatest monologue.Kirk: They used to say if man could fly, he'd have wings. But he did fly. He discovered he had to. Do you wish that the first Apollo mission hadn't reached the moon, or that we hadn't gone on to Mars and then to the nearest star? That's like saying you wish that you still operated with scalpels and sewed your patients up with catgut like your great-great-great-great-grandfather used to. 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! You may dissent without prejudice. Do I hear a negative vote?"
- There's one particularly awesome thing about this speech - at the time it was written, months before the script was submitted to the production team and a full year before this episode aired, Apollo 1 malfunctioned and killed all three astronauts. All the proceeding tests were unmanned, and the program itself was in danger of being cancelled. The writers of Trek were delivering this speech to all of humanity.
- 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.
- Gilligan Cut: "You're going to WHAT?"
- A God Am I: Apparently, Sargon's people developed this attitude as a result of their minds becoming so powerful, which led to the war that destroyed their world. Thalassa has a moment of this when McCoy tells her he won't trade Mulhall's body for Kirk's life ("You dare defy one you should be on your knees worshipping? I could destroy you with a single thought!") and uses her powers to torture him, but quickly becomes horrified with herself.
- Grand Theft Me: Subverted, as the crew actually let them, closer to Willing Channeler. They understand that Sargon and Thalassa only intend to do this temporarily, long enough to construct robot bodies to hold them instead. Henoch, unbeknownst to the others, has more permanent designs on Spock's body.
- Green-Eyed Monster: Henoch has more than a little of this.
- Kill the Host Body: Kirk resorts to injecting Spock's body with lethal poison to destroy Henoch. Subverted when it turns out that Sargon arranged for them to think that the hypo was deadly so that Henoch would flee and render himself vulnerable, and that Spock's consciousness was hidden within Nurse Chapel.
- Kissing Under the Influence: Sargon and Thalassa are a very Happily Married couple who 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. Yes, even by Shatner's standards.
- 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.
- Possession Burnout: Possessed bodies have their metabolic and heart rates shoot up to dangerous levels (presumably native to the aliens' original forms); Spock's Vulcan physiology can tolerate it for several hours, but the humans can only take it for a few minutes before risking death.
- Power Echoes: When the aliens possess humans they gain echoing voices.
- Psychotic Smirk: Henoch gives a few in Spock's body.
- Readings Are Off the Scale: Dr. Mulhall announces this while she and Dr. McCoy are scanning Sargon's underground sanctuary.
- 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.
- Right for the Wrong Reasons: McCoy naturally has his doubts letting Sargon, Thalassa and Henoch borrow bodies. His concern proves well founded, but not for the reasons he expected as the danger comes from Henoch refusing to leave Spock's body and plotting to kill Kirk's body to ensure Sargon's death.
- 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.
- There's a brief moment where McCoy sees Chapel leaving sickbay and yells "Nurse Chapel, what in the devil?!" — right after Spock's consciousness was stored in Chapel's body. That's him. Majel Barrett actually got to play Spock for those few seconds.
- 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.
- Together in Death: Sargon and Thalassa ultimately choose this, deciding that they do not fear oblivion so long as they are together.
- Willing Channeler: Essentially what they're all doing, although Roddenberry and Gene Coon insisted on removing anything that seemed the least bit "spiritual", to the point that author John Dugan, a devout Catholic, used his pseudonym Kingsbridge on this. Roddenberry had rewritten the final scene to say that the Arretians departed into "oblivion" rather than just deciding to go on existing without bodies in "eternity" or "infinity" as Dugan had wanted it. He was a university professor and all his students and colleagues knew his beliefs.
- Your Mind Makes It Real: Sargon used his abilities to make McCoy believe he loaded a hypo full of poison so that Henoch would believe it as well and flee Spock's body after Chapel injected him with it.