Original air date: April 13, 1967
For many years, a strange epidemic of insanity has slowly but surely made its way through an interplanetary system and the planet Deneva is right in the path. The Enterprise tries to contact a Denevan ship that seems intent on flying straight into the sun. The only broadcast they're able to get from the ship is the eerie chant of "Free! I'm free at last!" before the entire ship and its pilot disintegrates in the radiation of the sun.
Kirk, McCoy, Bones, Scotty and a trio of Redshirts (who miraculously survive the experience!) beam down to Deneva to find the streets nearly empty. They are forced to stun a small but nonetheless angry mob. They find that everyone here is either dead or half crazy. They visit the home of Kirk's brother, Sam. Sam is dead, his son is unconscious and his wife is screaming mad while trying to block up an air vent. It is soon learned that Deneva has been invaded by bat sized, unicellular creatures. While the Redshirts are spared their usual abuse, Spock is on the receiving end of an alien attack.
On board the ship, Kirk's sister-in-law dies screaming in pain and ranting madly. Spock is in pain so intense even his Vulcan mental training can't dampen it. Kirk and McCoy try desperately to find a way to kill these creatures.
Operation: Annihilate Tropes!:
- Air-Vent Passageway: How the creatures get into homes. Justified that they are small enough to get through the very small vents.
- All for Nothing: Bones is devastated upon learning that if he'd just waited a few minutes, he would have learned the experiment that blinded Spock wasn't necessary. Luckily there's a Deus ex Machina waiting.
- Almost Dead Guy: Aurelan Kirk regains consciousness just long enough for her to explain the situation with the parasites before the one infecting her kills her to keep her quiet.
- Body Horror: Imagine one of these parasites stinging you; then you are in constant agony and being manipulated by the collective intelligence who can increase the pain any time they want to bring you to heel or to simply kill you.
- Camera Abuse: One of the creatures flies at the camera.
- The Chains of Commanding: Once Kirk learns the cause of the mass insanity he tells his people he cannot allow the parasites to penetrate any further into the Federation, even if it means using the firepower of the Enterprise to bombard Deneva and kill the parasites even though it'll result in the deaths of the colonists.
- Chekhov's Gun: The stray Denevan pilot that the Enterprise attempts to contact at the start of the episode, screaming about being free while he flies into the sun. At first he seems like yet another insane victim of the aliens. Then it turns out he was screaming about being free from the aliens' influence, thanks to the sun being lethal to the alien...and sadly, the pilot as well.
- Civilization Destroyer: Going back to ancient times, a number of civilizations on different planets have been destroyed by outbreaks of mass insanity. The cause of the insanity is alien creatures that attack people and inject material into their bodies that takes control of their nervous systems. The aliens make the victims travel to other planets using starships, thus spreading the infection.
- Cool Shades: Averted. The goggles that block the blinding rays of light are actually kind of dorky looking.
- Deus ex Machina: Wow! Good thing Vulcans have extra eyelids!
- Died in Your Arms Tonight: A downplayed platonic example; Kirk cradles Spock, who has been left with serious injury and pain after being attacked and infected by one of the parasites.
- Disabled Means Helpless: Everyone seems to think this when Spock goes blind. The VISOR hasn't been invented yet and blindness is still serious business in the Trek world of this time. Presumably, Spock likely would have been able to adapt to it in time (had it not turned out to be temporary), but it's completely believable that he'd struggle initially, even if there was assistive technology available.
- Driven to Suicide: An unarmed Denevan pilot, driven mad by pain caused by the parasite he's infected by, flies his ship into the sun.
- Dying as Yourself: The Denevan who flies into the sun is freed from the creatures' influence just before his ship burns up.
- Facial Dialogue: Spock and McCoy have a brief Fascinating Eyebrow war. Spock wins.
- Hell Is That Noise: The creatures have two, the buzzing that they usually make and the shriek when they're agitated.
- Heroic Sacrifice: Spock doesn't hesitate to make one.
- Hive Mind: The creatures are described as being autonomous parts of a single organism. This doesn't go anywhere in the final version of the episodes, but in early drafts it was set-up for a denouement in which the creatures are defeated by locating and destroying the Hive Queen.
- Improbable Infant Survival: Hundreds of people, including Kirk's brother and sister-in-law are killed by the parasites, but Kirk's nephew Peter survives.
- It's Personal: Our heroes have faced weird alien menaces before, but this time it's personal because Kirk's family (and Spock) are among the victims.
- Lampshade Hanging: Yeoman Zahra, looking at the obviously inanimate rubber parasites: "It doesn't even look real."
- O.O.C. Is Serious Business: Spock, driven mad by pain, goes wild and tries to take over the ship. Also, several times (brilliantly played by Nimoy) Spock visibly twitched. Not grossly, but just to remind you that Spock is having to use all his Vulcan discipline to hold down the pain, and it can't last forever.
- Pietà Plagiarism: Kirk cradles the visibly shaken Spock after the Vulcan is attacked and infected.
- Playing Nice for Now: Inverted. Spock and Dr. McCoy bicker a lot, but it's revealed that, deep down, they actually like and respect each other. (Resulting in a rare Ho Yay moment for this pair.) McCoy is loathe to let Spock know this.
- Puppeteer Parasite: Parasitic creatures that resemble flying pancakes attack planetary colonists—and eventually Spock.
- Sadistic Choice: Kirk must choose between the deaths of several million people, including his first officer and young nephew, or allowing these creatures to continue being the scourge of the galaxy.
- Single-Episode Handicap: Spock is blinded after they blast him with "the complete spectrum of light" to rid him of a parasite that is UV-sensitive. It's okay, though, because Vulcans actually have third eyelids that somehow restored his sight by the end of the episode. This extra set of eyelids is thereafter never mentioned again...until the last season of Enterprise, anyways.
- Stock Footage: Stock footage of Leslie's hands from "The Alternative Factor" is used to represent the personnel in the satellite control room. This shot was removed from the remastered version of the episode.
- Survival Mantra: "I am a Vulcan. There is no pain."
- Take a Third Option: Kirk demands one, as he usually does when faced with a Sadistic Choice. He gets it!
- Uncanny Family Resemblance: No one comments on it, but Sam Kirk looks an awful lot like Jim Kirk with a mustache...
- Weakened by the Light: The blobby aliens turn out to be weak to close exposure to sunlight, or rather, one of the wavelengths that the local sun gives off. The Enterprise weaponize this knowledge with a network of satellites to kill all of the aliens without also killing or even merely blinding the human population of Deneva.
- What Happened to the Mouse?: We never see what happens to Kirk's nephew Peter, who had lost both his parents. Is he sent to live with distant relatives, perhaps Kirk's parents? Is he put in a children's home? (In a scene that was actually shot (production stills survive) but cut from the final edit, Kirk sees off his nephew Peter, who has been adopted by his father's business partner.)
- Wrench Whack: One of the mind-controlled human colonists ineffectually attacks the landing party with a large wrench.