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 is 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:
- Acting for Two: William Shatner played both Captain Kirk and his brother, Sam. Granted, for Sam all he had to do was lay there and look dead. Come to think of it, that's probably a challenge for Shatner!
- Shatner's brief appearance as Sam's corpse is the only time in the franchise that he does not play James T. Kirk or someone mimicking or inhabiting the body of James T. Kirk.
- Air Vent Escape: How the creatures get into homes.
- 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.
- Angst? What Angst?: The jocular tone of the epilogue is somewhat jarring, considering the deaths of Kirk's brother and sister-in-law are not even mentioned.
- Body Horror: Imagine one of these parasite's 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 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!
- Disabled Means Helpless: Everyone seems to think this when Spock goes blind. Guess the VISOR hasn't been invented yet!
- 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.
- Infant Immortality: Kirk's young nephew, Peter survives. His parents and half the planet aren't so lucky.
- Kill It with Fire: At first, this seems to be the only way to kill the creatures. Possibly because they look a bit like fried eggs? Intense light kills them too.
- Madness Mantra "I am a Vulcan. There is no pain."
- Out Of Character Is Serious Business: Spock, driven mad by pain, goes wild and tries to take over the ship.
- 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.
- Punctuated Titles: "Operation: Annihilate!"
- 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 gets over being blind pretty quickly.
- 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...