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.
Tropes for this episode include:
- 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.
- Artistic License Biology: Single-celled organisms the size of dinner plates are not going to happen; there's some hand-waving about them originating in a distant galaxy where science is not as we know it, but that doesn't excuse them surviving in this galaxy. Also, they have a lot of behaviors that are beyond the reach of a single-celled organism, like possessing detachable stings that work their way into the victim's nervous system.
- 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.
- 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. Guess the VISOR hasn't been invented yet! Justified in that the VISOR is new technology eighty-plus years later and Geordi explains it from time to time to shipmates who don't know what it is. And of course Science Marches On and Geordi replaces it with robot eyes just a few years later. In another episode (Is There In Truth No Beauty?) a blind woman wears a sensor web, but although it does much to offset blindness it does not let her see, and it is exotic technology. At this time in the Trek Verse, blindness is still serious business.
- Driven to Suicide: The Denevan pilot.
- 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.
- Infant Immortality: Kirk's young nephew, Peter survives. His parents and half the planet aren't so lucky.
- 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.
- Madness Mantra "I am a Vulcan. There is no pain."
- 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.
- 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.
- 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...
- 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.)