[[quoteright:350:https://static.tvtropes.org/pmwiki/pub/images/lights_of_zetar.png]]
[[caption-width-right:350:Ooh, sparkly demons from outer space!]]

'''Original air date:''' January 31, 1969

JustForFun/TheOneWith the ''Enterprise'' being attacked by evil ghosts. No, seriously.\\
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The ''Enterprise'' is transferring new equipment to Memory Alpha and [[GirlOfTheWeek Lieutenant Mira Romaine]] has been assigned to supervise. Scotty himself is doing quite a bit of supervising the new lieutenant himself. Seems he's finally over [[Recap/StarTrekS2E2WhoMournsForAdonais Lt. Palamas.]] She isn't discouraging it. Suddenly, it's disco inferno! An energy storm creates a flashy light show worthy of Vegas that destroys all the inhabitants of Memory Alpha and wipes out its computer banks. It's the destruction of the Library of Alexandria [[RecycledInSpace IN SPACE!]] Mira, meanwhile, appears to be having an epileptic seizure brought on by the sparkly lights. Or is it more than that?

Particularly notable for its writers, ventriloquist Creator/ShariLewis and her husband Jeremy Tarcher.

Trivia: The Star Trek Wiki is called "Memory Alpha" after the inter-galactic library mentioned in this episode.

!!The Tropes of Zetar:

* AssimilationPlot: The eponymous creatures are a floating energy hive mind comprised of the last members of a doomed civilization. Their goal is to find a compatible human to merge with so they can regain physical sensations and abilities.
* AuthorAppeal: The Zetari control Mira's actions and speak through her. Almost as if [[PeoplePuppets she were a puppet....]]
* AuthorAvatar: Mira supposedly started out as a very obvious one for Creator/ShariLewis, though script consultant Arthur Singer pared this back to the point where she has a much more generic personality. Indeed, Lewis had hoped to play Mira herself.
* BriefAccentImitation: Nurse Chapel puts on a Scottish brogue when praising Scotty's bedside manner for his lady love.
-->''Ye're in the wrong business!''
* CassandraTruth: Once again, Scotty nearly dooms everyone by scoffing, this time at the possibility that Mira might be psychic.
* DeadpanSnarker: Kirk gets a chance to snark, aiming mostly at Scotty, who's finally getting some. He ends the episode by pointing out an ''Enterprise'' first. [[VitriolicBestBuds Bones and Spock]] agree with each other!
* DemonicPossession: The sci-fi version happens to Mira (and the technician on Memory Alpha, though she doesn't survive the experience). The creepy throaty sounds they both make when affected by the Zetars emphasize this.
* DeusExMachina: The reason the pressure chamber works is basically "We had the prop."
* DrJerk: An inversion! Mira is Patient Jerk, much to Bones' consternation.
* EeriePaleSkinnedBrunette: While not particularly eerie (at first), Mira does have dark hair and fair skin. Possession by the Zetarians pushes her deeper into the trope.
* EyeMotifs: A lot of close ups are shown of Mira's eyes, showing reflections of the glittering lights, her flashes of the future and her fight against the possession of the Zetarians.
* FaintingSeer: Mira becomes one.
* GirlOfTheWeek: Notable because she's Scotty's.
* LittleNo: Mira to Scotty when the ''Enterprise'' begins firing on that [[NegativeSpaceWedgie flashy light thing]].
* LoveMakesYouDumb: Scotty's feelings for Mira clearly have him distracted; at one point, he's hovering over her in Sickbay when Kirk expects him to be in Engineering.
* MadeOfIron: Mira survives more than 30 atmospheres in the pressure chamber. At least they remember to commence decompression ''very'' slowly...
* MayDecemberRomance: Scotty used to work with Mira's dad. Kirk just rubs in the age difference by often referring to Mira as "the girl".
* MonkeyMoralityPose: Not shown, but very subtly demonstrated. During the light storm, Uhura is unable to respond to a hail from Starfleet (Hear no evil), Chekov cannot move his eyes to see how to set in a new course (See no evil), and Sulu is unable to speak (Need I say more?).
* OurGhostsAreDifferent: The Zetars were once humanoid, but when their physical bodies were destroyed by a disaster on their homeworld, the last few of them clung to life (...[[HandWave somehow]]) and have been drifting around the galaxy as EnergyBeings of some sort. They are insistent that it is not their time to die, and have no qualms about taking Mira's life so they can live out the rest of theirs.
* PietaPlagiarism: Scotty holds Mira many times in this style. When the Zetarians zap Scotty, Kirk holds him like this.
* ThePowerOfLove: It is speculated that the pressure used to drive out the Zetari may have been aided by Mira's love for Scotty.
* PowersViaPossession: Not only does Mira get the ability to see the future through possession by the Zetari, but her brain wave pattern is changing to match theirs.
* SickeningSweethearts: Scotty can't seem to take his eyes or his hands off Mira. Kirk seems mildly annoyed by their goofy obsession with each other.
* SpaceMadness: Scotty says that going on your first deep space trip can affect a person's mind.
* StockFootage: The wide overhead shot of the bridge at the beginning of the Teaser is recycled from "[[{{Recap/StarTrekS1E16TheGalileoSeven}} The Galileo Seven]]". Lt. Hadley is sitting at the navigation station instead of Chekov, and the crew are wearing the velour fabric tunics from the first two seasons.
* {{Synchronization}}: When the Zetarians are hit by a phaser attack, Mira collapses in pain.
* TortureAlwaysWorks: When the Zetari don't respond to the KirkSummation, Kirk drives them out of Mira's body by putting her in a pressure chamber.
* WeComeInPeace: Kirk tells the Zetarians "We wish you no harm." Too bad the feeling isn't mutual.