Original air date: December 15, 1967
It's just another day of exploration for Kirk and company. There's a planet called Argus X to be explored, a new mineral is discovered, a mysterious cloud is killing Redshirts. (The Redshirt body count for this entire episode is six, including one off screen.) The victims of the mysterious cloud of smoke are sucked dry of all blood, but with no visible cuts or punctures. Kirk remembers an incident like this from 11 years ago when he was just a Lieutenant on his first tour of duty. The guilt of the memory starts to affect Kirk. If he had just fired his phaser a little more quickly all those years ago, Captain Garrovick, half his crew and now these crew members wouldn't be dead.
Speaking of Captain Garrovick, the new ensign aboard the Enterprise is also named Garrovick...and it's no coincidence. Ensign Garrovick is the son of the late Captain Garrovick. When Ensign Garrovick is first confronted with the gaseous form, he makes the same mistake Kirk made all those years ago by hesitating to fire. Kirk, rather than sympathizing, becomes irate with the young man. Kirk develops an insatiable obsession with destroying this creature.
Can Kirk take down this blood thirsty monster without becoming a monster himself?
- Absurdly Ineffective Barricade: The shot of the creature pouring through the vent around Spock's hands with which he's futilely trying to block it.
- Air Vent Infiltration: How the cloud creature gets into Garrovick's quarters. Being a mist, it doesn't even have to take the grid off.
- Almost Dead Guy: Kirk interrogates a dying Rizzo as to what he saw and sensed, however the results are hardly reliable—Bones points out that the drugged and semi-conscious patient could just as easily have been telling Kirk what he wanted to hear.
- Anti Matter: Phaser fire has no effect on the vampiric cloud. Nothing less than a matter/antimatter bomb will destroy it. It will be difficult to destroy the creature without destroying whoever sets the trap for it.
- Artificial Gravity: An anti-gravity device is used to carry the magnetic vacuum chamber of anti-matter, so it can be portable by just two men.
- Artistic License Chemistry: Early on, Kirk suggests (correctly, as it turns out) that the reason they can't detect the dikironium creature is that it knows they are looking for it and can somehow hide itself. Spock replies that "it would have to be able to change its molecular structure, like gold changing itself to lead or wood changing itself to ivory." The second, yes, but the first would require a change on the sub-atomic level, not the molecular one, and Spock of all people should know that.
- Artistic License Physics: An ounce of matter, converted entirely into energy, releases a blast of about one megaton. A city killer, yes, but hardly "ten thousand cobalt bombs" or sufficient to rip away a planet's atmosphere.
- The Bait: After the cloud vampire chugs down that jar of blood faster than they thought it would, Garrovick attempts to knock Kirk out and take his place as the bait. Kirk recovers fast enough to tell him "consider yourself on report" and carry out his original plan to beam out right before detonating the bomb.
- Black-and-White Morality: Unlike in "The Man Trap", there's no sympathy here for the Monster of the Week. Kirk describes it as a evil and malevolent force that must be destroyed (though he's not presented as being very objective at the time), even though it's only trying to feed. However unlike the salt vampire, the ability of the creature to travel interstellar distances makes it a threat that Starfleet can't ignore.
- Bothering by the Book: Spock and McCoy formally express their concerns about their captain's behavior in the regulation Starfleet manner, even down to the words being used.
- Captain's Log: Averted with Kirk doing a Personal Log, given that he's expressing doubts about himself in them.
- Chekhov's Gun: Scotty informs the captain he's been cleaning the radioactive disposal vent on number two impulse engine. Later Kirk orders Uhura to ignore an open hatch warning from that same vent, but the creature uses it to enter the Enterprise.
- Death by Transceiver: Rizzo's party has encountered the vampire cloud.Rizzo: [Activates communicator] Captain?
Kirk: Kirk here.
Rizzo: Captain, there's a strange cloud, sir. [It reaches him and he starts choking] Cloud... cloud...
Kirk: Fire into it immediately!
Rizzo: [Heard over communicator] Help me. Help! [Dies]
- Death Is Cheap: Lt. Leslie dies. Hes back on the bridge before the end of the episode. His return is never explained on screen. (A deleted scene had McCoy reviving Leslie and the other victims of the cloud.)
- Deer in the Headlights: Kirk and Garrovick both froze in fear the first time they saw the mysterious cloud. Eventually, they learn that it didn't matter anyway. Phaser fire has no effect on the cloud.
- Deus ex Nukina
- When the creature gets into the ventilation system, the crew decide to flood the system with radioactive waste! No mention is made of how they decontaminated the vents afterwards.
- An anti-matter blast powerful enough to rip away the planet's atmosphere is the only way to be sure they've killed the creature. One assumes Spock did scan for lifeforms first.
- Doctor's Orders:
- Invoked by Nurse Chapel to get Garrovick to stop feeling sorry for himself and start eating. She claims that Bones has sent taped orders with only one word: EAT. Turns out she's bluffing about that.
- Bones threatens to file a medical report saying Kirk is unfit to command, through he needs a command rank officer to back that up—in this case, Spock. It's implied he won't go through with it as they just want Kirk to reconsider his actions, but it's played quite seriously at the time.
- Dodge the Bullet: We're told this is why the cloud is immune to phasers, getting out of the way impossibly quickly (although the effects don't reflect this at all). This leads both Garrovick and Kirk to kick themselves about how the hell they managed to miss a giant cloud at short range. Spock works out that the creature can throw itself out of time sync the instant before a photon torpedo hits. The only way to destroy it is with massive overkill using an antimatter bomb that will destroy most of the planet as well.
- Et Tu, Brute?: Kirk has this look when Spock walks into his quarters to back up McCoy.
- Evil Smells Bad: Inverted - Both Kirk and one of the redshirts who survives says the smell the creature gives off is cloying, almost like being smothered in honey or molasses.
- Fantastic NukeGarrovick: Just think, Captain, less than one ounce of antimatter here is more powerful than ten thousand cobalt bombs.Kirk: Let's hope it's as powerful as man will ever get.
- Follow in My Footsteps: Ensign Garrovick joined Starfleet just like his father before him.
- Gut Feeling: Unlike other examples of this trope in Star Trek, this is presented as being highly subjective and dangerous to rely on (though as usual The Captain is ultimately shown to be right in his instincts).
- Heroic BSoD: Garrovick, after being confined to quarters.
- Heroic Sacrifice: Played with. Spock shoves Garrovick out of the room when the cloud vampire pours in through the air vent. While it's true that Spock's Vulcan blood was instrumental in defeating another vampiric creature in "The Man Trap", he was taking a risk that it would work this time. Of course, he survives. Garrovick tries to knock out the captain, believing that Kirk's plan is a Heroic Sacrifice, but his attempt fails.
- He's Dead, Jim: Inverted; this time Jim says they're dead without checking them, but Spock scans the redshirts and finds that Rizzo is still alive.
- Hospital Hottie: Nurse Chapel in her miniskirt. Even that's not enough to rouse Garrovick's spirits.
- The Hunter Becomes the HuntedSpock: May I suggest that we no longer belabor the question of whether or not we should have gone after the creature? The matter has now been rendered academic. The creature is now after us.
- I'm Cold... So Cold...: A half-conscious Rizzo remembers being cold when the cloud creature attacked him.
- Immune to Bullets: The creature can't be killed by a phaser set to Disrupter B or even a photon torpedo, thanks to its ability to go out of sync.
- I Need a Freaking Drink: When Spock comes to Bones and says "I need advice." Bones responds "I need a drink!" as a You Have GOT to Be Kidding Me! response.
- It Can Think: The first thing that convinces Spock that the cloud creature is actually a creature of at least semi-sentient nature is how it was capable of reacting to the Enterprise's attack in space and then turning around and attacking in retaliation. The audience sees this trope when the creature first appears, only to quickly hide when Spock fires a phaser (at a rock to gather an ore sample).
- It's All My Fault: What Kirk thought for years and Ensign Garrovick thought for a bit for their Deer in the Headlights hesitation facing the cloud vampire. As it turns out, phaser fire is useless against it so they have nothing to regret.
- Let's Split Up, Gang!: Justified examples—Kirk sends his security team off to sweep the area or try a pincer movement. Rizzo and later Garrovick are a few yards away from their companions when they are attacked, but all are within sight of each other.
- Lost Aesop: The episode is a monster-hunt story that revolves, for the most part, around Kirk's titular obsession with the monster. When the creature first attacked him and the ship he was serving on, 11 years earlier, he hesitated to fire at it and the creature killed half the ship's crew. In the episode itself, a young security officer on the Enterprise also hesitates when faced with the same creature, and the creature ends up killing several men. Both Kirk and the young officer blame themselves for their crewmates' deaths, and there is plenty of angst over the matter. How is this solved? Turns out that the creature is immune to phasers, and neither of the two men could've stopped it when they had the chance. The Aesop that was being set up is that "humans hesitate by nature, sometimes it can't be helped, and you can't spend your life blaming yourself for it". This is even outright explained by Spock. However it ends up being something like "failure is sometimes okay in hindsight" — which is no Aesop at all. Needless to say, once the creature is revealed to be nigh-invulnerable, the episode proceeds with the monster-hunt and never touches on any of the above in any way.
- Ludicrous Speed: The ship goes to Warp 8 when pursuing the creature, until Scotty is able to convince the captain that the ship will blow up if he doesn't slow down. Fortunately the creature decides to turn and fight at that point.
- Moby Schtick: Not the first or last time that Trek would go to the well of Melville, given this trope's use in The Doomsday Machine, and of course, Star Trek II: The Wrath of Khan and Star Trek: First Contact. However, this version of the story has the crew getting all the evidence needed to convince them that Kirk is right that the creature is a catastrophically deadly menace to the galaxy that has to be stopped immediately and do everything they can to help Kirk. In addition, the desire to destroy the creature is driven by Survivor's Guilt rather than revenge.
- More Expendable Than You: Garrovick attempts this, only for Kirk to get the better of him and explain sacrificing himself is still plan B.
- My Greatest Failure: How Kirk views his last confrontation with the cloud creature.
- Non-Standard Prescription: When Garrovick is brooding on his feeling of guilt, Nurse Chapel tells him that McCoy has issued a prescription for him that consists of the single word "Eat". However, it turns out she made this up.
- OOC Is Serious Business: As soon as he is confronted by the cloud vampire for the first time in 11 years, the usually genial Captain Kirk starts acting like he had an extra helping of bitch flakes.
- Our Vampires Are Different: The cloud creature drinks blood, but does it without breaking skin, or cracking glass in the situation with the jar of blood. It is always in the form of a cloud of white smoke. Incidentally, many vampires of legend were able to take a gaseous form.
- Permission to Speak Freely: It is established that Starfleet Regulations give the First Officer and Medical Officer the right to confront the Captain if he is behaving in ways that they feel put the ship and her crew in unwarranted danger.
- Psychic Link: Kirk has developed some sort of connection to the creature which he can't explain, one which allows him to accurately judge that it's intelligent and even to guess its motivations. During his brief moments of consciousness, Rizzo expresses something similar.
- Quit Your Whining: Garrovick had been confined to quarters and made to think It's All My Fault. Chapel brings him a meal and when he refuses to eat tells him its Doctor's Orders and he should stop wallowing in self-pity. Spock also tries to rouse his spirits, though the creature interrupts events. Spock apparently sacrificing his life because Garrovick accidentally knocked the vent open doesn't help matters. Fortunately, Spock comes out perfectly fine and Captain Kirk is not the only one relieved at that.
- Race Against the Clock: The Enterprise has to rendezvous with another starship to pick up and then deliver some time-sensitive vaccines. The crew's concern is based on the fact that Kirk seems to be ignoring this.
- Red Alert:
- Called verbally down on the planet when Kirk first senses the creature and alerts his redshirts.
- A red alert serves as the Conflict Killer when Kirk's senior officers confront him.
- Garrovick hears the call to battle stations and This Is Not a Drill, so he goes to the Bridge to ask to be allowed to take his battle station. Kirk is too busy to pay attention at the time, but later returns him to duty.
- Redshirt Army: This episode has the highest body count for them. One of the redshirts killed in the teaser is Lt. Leslie — but he gets better; he can be spotted wandering around the ship later the same episode, and by the next episode filmed ("The Immunity Syndrome") he is back on the bridge. The actor was Shatner's stand-in for the entirety of the series.
- Reverse Polarity: Justified; here it just involves reversing the flow of air through the vent, drawing the creature back into it.
- Rewind Gag: Played seriously as a special effect when the creature first appears and starts moving towards Spock, only to quickly retreat behind the rock when it realises Spock is armed.
- Sci-Fi Writers Have No Sense of Scale: In real life, one ounce of antimatter would have the explosive yield of a little over a megaton, not nearly enough to wipe out half of a planet's atmosphere.
- Scare Chord: Apart from obvious ones when the monster attacks, there's one when Kirk accuses his officers of conspiring behind his back when they're addressing their (quite justified) concerns to his face.
- Screen Shake: When the photon torpedo explodes and when the shockwave from the anti-matter explosion hits the Enterprise.
- Screw the Rules, I Have Connections!: Discussed early on with Ensign Garrovick. Despite being the son of Kirk's first commander, he says that he doesn't expect special treatment. Kirk, of course, gives him none.
- Shapeshifter: The creature can change its molecular structure, making it not only difficult to kill but difficult to pick up on scanners.
- Super-Speed Reading: Spock reads between eight to ten hours of recorded tape and provides a summary for Bones, who has been too busy autopsying dead redshirts to do more than scan through it.
- Survivor Guilt: Kirk holds himself responsible for the deaths of two hundred people on the USS Farragut, including the ship's captain, because while manning the phaser array he hesitated for a moment before firing. This causes him to lash out at Garrovick who did the same, despite Spock pointing out that it's an entirely normal reaction when facing an unknown situation.
- Tap on the Head: Averted. Ensign Garrovick tries to knock Kirk out in order to sacrifice himself, but it doesn't work.
- Teleportation Rescue: Played for even more suspense than usual. Kirk not only waits till the very last second before ordering Spock to beam him out, thanks to the anti-matter explosion there's some question as to whether they can successfully materialise at the other end.
- Tell Me About My Father!: After Ensign Garrovick and Captain Kirk succeed in making the cloud vampire go 'splodey, Kirk offers to tell Garrovick some Tall Tales about his father. Garrovick readily accepts.
- That's an Order!:
- Time for Plan B: Due to his Alien Blood, Spock points out that it's logical for him to take the dangerous task of planting the anti-matter bomb. Kirk uses this trope to get him to stay on the Enterprise instead; if his plan fails, he's counting on Spock to come up with a plan that doesn't.
- This Is Something He's Got to Do Himself: Kirk insists on planting the anti-matter bomb himself despite the risk, and he willingly accepts Ensign Garrovick's offer to accompany him, knowing he has the same motives.
- Too Spicy for Yog-Sothoth: The monster finds Spock's copper-based Vulcan blood inedible, and flees. McCoy wryly jokes that Spock "must have left a bad taste in its mouth" which Spock says is figuratively true.
- Trapped in Containment: An example where the trapped individual does it himself. Spock shoves Lt. Garrovick out of the room and shuts the door when the vampire cloud leaks in. Kirk agrees with the action and refuses to let the crewmen with him open the door lest the creature get out and kill even more people. To everyone's relief, Spock comes out unharmed a few minutes later; the creature found his blood unappetizing.
- Trauma Button: Kirk gets really tense when he smells a familiar sweet aroma and sends his men on a search-and-destroy mission.
- What the Hell, Hero?: Bones seems to have a talent for "What the Hell?" speeches. He gives one to Kirk when he believes that the captain's zeal for destroying the monster has become an unhealthy obsession.
- You Are Better Than You Think You Are: While Kirk as a lieutenant on the USS Farragut was kicking himself for hesitating firing on the cloud creature in the first encounter, that ship's Executive Officer recorded in his log that Kirk had performed "with uncommon bravery". Kirk then gives a similar sentiment to Ensign Garrovick, pointing out that even if he had fired his phaser on time and on target it would have made no difference.
- You Are in Command Now: Ensign Garrovick takes the position of Rizzo after he is killed.
- You Called Me "X"; It Must Be Serious:
- When McCoy addresses Kirk as "Captain" instead of "Jim" after Kirk first returns from Argos X, it's the first sign that McCoy is starting to doubt Kirk's judgment. Later, he visits Kirk privately and starts a conversation about his past encounter with the creature and his current behavior. Kirk suggests he's pushing their friendship too far, and McCoy says, "I'm not, Jim. This is professional, Captain," before bringing in Spock for an official inquiry.
- From the opposite side, Spock calls Kirk "Jim" while assuring him that what happened to the Farragut wasn't his fault.