The Enterprise has arrived at the ice planet Psi 2000 to document its impending collapse. Spock and some other loser beam down in Hazmat suits to investigate why a laboratory on the planet shows no life signs: turns out, everybody froze to death because someone left the door open and nobody cared enough to correct that. While Spock is investigating the corpses, his companion unwisely removes his glove to scratch his nose, allowing a strange red fluid to drip onto his hand.
Back on board the Enterprise, the Red Shirt starts acting strangely, in turns belligerent and depressed. He gets into a fight with Sulu and a guy named Lt. Kevin Thomas Riley, brandishes a dinner knife at them, and mildly injures himself. However, when McCoy operates on him, he dies, seemingly losing his will to live. McCoy is perplexed, as the victim was in good health and was previously known as a strong-willed individual. The reason for his odd behavior soon becomes apparent: he carried a virus that removes inhibitions (and makes one extremely sweaty, incidentally).
Chaos engulfs the ship as the virus spreads: crew members act carelessly, laughing maniacs run around the halls painting creepy messages in red paint, Sulu menaces passersby with a fencing sword, and Spock enters a crippling depression. Worst of all, Riley has locked himself in the engine room, hijacking the PA system to make bizarre announcements and advise women on hair and makeup styles, stranding the ship just as the planet is breaking up, threatening the Enterprise. Kirk and Scotty try to break into the engine room while McCoy cooks up a vaccine. However, by the time Scotty gets the door open, it's revealed that the engines were stopped completely and that restarting them would take more time than the planet has. Desperate, Kirk orders Scotty to... do something involving matter/antimatter annihilation to get the Enterprise out of the way. Scotty cries that it only has a 1-in-10,000 chance of not blowing them to smithereens (so, naturally, it will work perfectly), but he needs Spock's help. Kirk tries to slap Spock out of his funk, but Spock only recovers when Kirk empathizes with him under the virus's influence and reveals the heavy burden of being Captain of the Enterprise—contrary to popular belief, you can never get laid, at least not with crew members. McCoy cures the crew and the ship escapes, but with a strange side-effect: it travels about three days backwards in time, in the series' first instance of time travel.
- Anguished Declaration of Love: Nurse Chapel makes one for Spock.
- Artistic License Astronomy: Why would a planet break up due to it's sun going dark? Also, the universe isn't old enough for white dwarves or any other star to go dark, either, if it is meant that the star turned into a black dwarf.
- A 'dark star' was an early name for a black hole, so it's likely that's what they're referring to, with the planet breaking up as it falls into the black hole's gravity well.
- Broken Tears: Spock loses all control over his emotions.
- Clothing Damage: A bizarre instance; it's already been established that hyposprays can be used to inject people through their clothing, and even if they did need to be used on bare skin, McCoy could have just injected Kirk on the neck (which isn't covered by his uniform), or have him roll up his sleeve. Instead, for absolutely no explicable reason, McCoy tears open Kirk's uniform at the shoulder in order to inject him.
- Interestingly, it kind of looks like it was accidental. McCoy seems to only lightly tug at Kirk's collar and suddenly the entire sleeve comes off. Likely a good example of the low costume budget.
- Curtain Clothing: The environmental suits were made from Art Deco shower curtains.
- Cute, but Cacophonic: Lt. Riley. "I'll take you hoooome Kathleeeeeeen!"
- Deadpan Snarker: Spock, of all people, gets in one when he tells two bridge crewmen to, "Take D'Artagnan here to Sickbay."
- Death by Despair: A Red Shirt tries to commit suicide, is stopped before he seriously hurt himself, then dies anyway because he can't summon the will to fight off the minor infection of the wound.
- Decontamination Chamber: The transporter room is shown to have the ability to decontaminate the outside of isolation suits with some sort(s) of radiation. Of course the sort of radiation that would do that would also, at the least, damage the skin of the people in the suits unless the suits blocked the rays...and unfortunately the idiot who beamed down with Spock had taken off a glove, been contaminated, and then put the glove back on—thereby making sure the rays would do nothing (not that they'd have done anything against an infection within someone's body anyway).
- Denied Food as Punishment: Riley tells Uhura she can't have ice cream because she interrupted his song.
- Driven to Suicide: Joe Tormolen, who laments that humanity doesn't belong in space, stabs himself, and then despite receiving prompt medical attention, loses the will to live.
- Drunken Song: Riley sings an "ancient Irish favorite."
- Early Installment Weirdness:
- Lampshaded. Lt. Riley claims that Sulu's interest in botany in "The Man Trap" was one of his weekly fads. This week it's fencing.
- They're still working through Spock's mannerisms by this time. When he nerve pinches Sulu, he says to the security guards, "Take D'Artagnan here to Sickbay."
- The opening Captain's Log doesn't include a Stardate, though the subsequent log after the opening credits does.
- Everybody Must Get Stoned: A disease makes everyone get progressively drunker.
- Get Ahold Of Yourself Man: Kirk attempts to slap Spock back into a steady state of mind. All it does is get Spock mad enough to slap him back, with enough force that he goes flying across the table.
- Getting Crap Past the Radar: Uhura's quip when Sulu grabs her.Sulu: Aha!, fair maiden!
Uhura: Sorry, neither.
- Graceful Loser: Riley has made a number of announcements over the ship's PA system, including a formal dance in the ship's bowling alley. After going through considerable effort to force their way into Engineering, Kirk leads the charge with Scotty and a pair of Security crewmen. Riley looks up from the chair and dryly observes, "No dance tonight."
- Hollywood Tone-Deaf: Kirk might not mind Riley's singing so much if it had somewhat less enthusiasm and a little more musical talent.
- The Hyena: The disease turns a few people into this.
- Inelegant Blubbering: Spock when he gets affected.
- Love Is in the Air: The disease makes people lose their inhibitions, so many of the afflicted find themselves acting more amorous than usual.
- Madness Mantra: Spock tries to ward off the effects of the disease by repeating "I am in control of my emotions" and reciting multiplication tables.
- Million-to-One Chance: Scotty gives the odds of successfully cold-starting the engines without blowing themselves up as 1 in 10,000—and that's with a supercomputer and weeks to calculate the formula. Fortunately, Spock is better than a supercomputer, if they can get him to stop crying long enough to focus.
- Murder Water: Contrary to the episode synopsis you just read, the episode itself doesn't have the cause to be a virus—that was the retcon used in the "revisiting" to cover the fact that the central premise of "The Naked Time" fell squarely into Science Marches On: polywater, a kind of high-viscosity, low-freezing point water that would tend to make other water into its own type. It's not actually sentient, but it is extremely dangerous, can move in ways that should be impossible, and is (a type of) water.McCoy. It's water! Somehow on this planet, water's changed to a complex chain of molecules. That's how we missed it. It passed from man to man through perspiration. Once in the bloodstream, it acts like alcohol... depresses the centers of judgment, self-control.
- Oh, No... Not Again!: Kirk's reaction to Lt. Riley's announcement that he is going to sing another rendition of "I'll Take You Home Kathleen."Riley: And now, crew, I will render "Kathleen"... ooonnne moooore tiiiimme!Kirk: [muttered] Please, not again.
- Oireland: Riley goes über-Irish while drunk.Riley. You know what Joe's mistake was? He wasn't born an Irishman.
- Orbits Do Not Work That Way: There is no ice-planet's reason in hell that the Enterprise needs to "spiral down" as the doomed planet shrinks. Are they connected by fishing line or something?
- The Plague: The disease is apparently non-fatal, but is highly contagious and, in both episodes, adversely affects the crew in the midst of a disaster which could destroy the ship.
- Plot Hole: Dr. McCoy, a front-line medical worker, never succumbs to the disease—pretty impressive considering that it manages to afflict his head nurse.
- Running Away to Cry: Spock locks himself away when the virus breaks down his emotional control.
- Schmuck Bait: What exactly did Kirk expect to happen when he touched the end of Sulu's sword?. (There are no euphemisms in the previous sentence, but don't let that stop your imagination.) To be fair, foils use in sport fencing don't have sharp tips because it's dangerous.
- Scotty Time: "I cannae change the laws of physics! I've got to have thirty minutes!" In this case, it's actually not an inflation, but the faster alternative of trying to cold-start the Enterprise's engines is way too risky for Scotty's skill level.
- Sequel Hook: Spock accidentally invents time travel, with Kirk noting that it could be pretty interesting to try again some day. The original plan was for this episode to lead directly into Tomorrow is Yesterday, but scheduling didn't work out.
- Shirtless Scene: Sulu gets one. Oh, my!
- Too Dumb to Live: Taking off your glove, getting some weird liquid on your hand, and then rubbing your nose? If the virus hadn't gotten him, this guy doubtlessly would have found some other way to kill himself in a few days.
- World of Ham: Everybody under the influence of the virus starts gobbling up the scenery, nobody better than Riley, though.