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The Enterprise has sent a landing team to a planet to search for tritanium deposits. While surveying, Captain Kirk notices a sweet honey smell and has his men go on alert. He tells them to scan for dikironium and immediately fire at anything that looks like a gaseous cloud. The landing party fans out to explore.

One group picks up a signal but loses it, at which point they are attacked. Kirk and Science Officer Spock rush over to the scene, finding three crewmen down; two dead, the other, Ensign Rizzo, near death. All of the men have had their red blood cells drained of hemoglobin. Kirk fears that the killer is the same entity he had encountered 11 years before while serving aboard the USS Farragut. As a young lieutenant, Kirk had hesitated in firing the ship's phasers at the creature, which attacked. The encounter killed half the Farragut's crew, with Kirk and the ship's first officer among the survivors.

The landing team returns to the Enterprise. Once aboard, Kirk becomes obsessed with finding this entity, feeling guilty that it caused so much damage and then eluded him. He ignores his priority orders to rendezvous with the USS Yorktown, to transfer urgent medical supplies destined for Theta VII, until Chief Medical Officer Dr. McCoy can confirm the cause of the men's deaths. Kirk tells him to reference the Farragut's medical logs. On the bridge, Kirk has Spock look for a gaseous entity that is able to conceal itself and pass through solid matter.

Kirk calls one of the security officers, Ensign Garrovick, to the bridge. The ensign's father was the commanding officer of the Farragut and lost his life to the entity. Kirk believes Garrovick's son will be as driven in finding the creature as he is and assigns him to the next landing party to hunt the entity down.

Kirk orders his landing party to fire at the entity as soon as it is spotted. The team spreads out, and Ensign Garrovick encounters the creature. He aims his phaser but hesitates, becoming frozen with fear as his partner is killed. The creature moves off just as Kirk arrives. Back on the Enterprise, Garrovick admits that he hesitated before firing his phaser and is confined to quarters.

McCoy confronts the Captain on his reckless obsession over the creature. Kirk justifies his actions by maintaining that the creature is an intelligent predator and a serious threat to inhabited worlds. McCoy backs off as Ensign Chekov reports that he is tracking the cloud creature moving away from the planet.

The landing party returns to the ship, and Kirk orders a pursuit of the cloud at Warp 8. The ship gives chase, but Chief Engineer Scott doubts the Enterprise can sustain the high speeds for long. Kirk orders a reduction of speed, and the entity slows as well. Kirk orders battle stations as the entity cloud comes into range and gives the order to fire their weapons. The phasers and photon torpedoes pass through the being. Not only do the weapons have no effect, but neither do the deflector shields - the creature passes through the ship's hull and into the ventilation system. Kirk orders Scotty to close all vents and flood the system with gas.

The creature exits the vents and kills two crewmen. It then enters the room of Ensign Garrovick and tries to attack him, but Spock was present shortly before it arrived. The creature gets a sample of Spock's Vulcan blood, which repels it due to its alien composition. The creature lets Spock go and flees the ship.

The creature makes its way to the fourth planet in the Tycho system, and the Enterprise heads there in pursuit. Kirk and Ensign Garrovick beam down to the planet with a case that holds an antimatter bomb. The two lure the creature with blood from the ship's medical supply, but it gobbles the "bait" before they can lead it to the bomb.

Kirk uses himself as bait and remain behind to detonate the bomb. He orders Garrovick back to the ship, but the officer refuses to abandon his captain. The two ready the bomb as the creature draws near. With the creature ready to envelop them, Kirk and Garrovick beam away and the antimatter bomb explodes, annihilating the entity.

Safe aboard the ship, Kirk takes Garrovick aside to thank him and tell him about his father, Captain Garrovick.[1]

Errors and ExplanationsEdit

The Nitpicker's Guide for Classic TrekkersEdit

Plot OversightsEdit

  1. The Cloud Creature is depicted entering the Enterprise through a jammed impulse engine vent, opened to the vacuum of space. As it is also depicted as unable to get through solid material, the only way it could have reached the interior would have been to open the duct or manifold corresponding to the jammed impulse engine vent to the ship's interior. This act would have depressurized the Enterprise, killing all aboard. The depressurisation could have been prevented by automatic activation of one way valves in the system.[N 1]
  2. For a moment, Spock seems to forget the basic properties of a gas. As the creature invades Garrovick’s quarters through a ventilation grille, the Vulcan puts his hands over the opening. Of course, the creature simply flows around them. He was making an educated guess that his copper based blood would repel the gas creature.

Changed PremesisEdit

  1. McCoy refers to Kirk as “Lieutenant” during his time on the USS Farragut. Kirk says that he served with Captain Garrovick (Ensign Garrovick’s father) from the day he left Startleet Academy. Yet in “Court Martial," Kirk says that—as an ensign-—he discovered a mistake made by Finney while serving on the USS Republic. It Kirk left the Academy and rose to the rank of lieutenant under Captain Garrovick, there are a limited number of ways to explain how he could be an ensign on the Republic. Did Garrovick switch from the Republic to the Farragut and take the young Ensign Kirk with him? Or did Kirk get busted back down to ensign after he left the Farragut’? Or do midshipmen at the Academy somehow achieve a rank of ensign and spend their summers in space? (While this last explanation seems reasonable, Starfleet is built on the Navy model with respect to rank. It is generally understood that the rank of ensign isn’t assigned until after you graduate from the Academy. Maybe Roddenberry decided to change this. Then again, maybe the creators just forgot what they said about Kirk in “Court Martial“!) See my comment on this in the entry for Court Martial.

Equipment OdditiesEdit

  1. The small tower that Scott admired in “I, Mudd,” has made it aboard the Enterprise and resides in sick bay! On the android planet, the chief engineer walked around it and was amazed at its construction. Perhaps it just looks like the tower from I, Mudd [N 2]
  2. Alter beaming Kirk aboard, Spock opens a communications channel to the bridge so the captain can issue some orders. Oddly enough, the little light on the speaker mounted to the top of the transporter console stays off. It probably suffered some minor damage during the effort to expel the gas creature.

Nit CentralEdit

  1. Keith Alan Morgan on Saturday, April 17, 1999 - 5:59 am: Scotty took the liberty of dumping radioactive waste? What about the planet below? Isn't it a violation of the Prime Directive to pollute a planet with radioactivity? Wouldn't that depend on where he dumped it?
  2. Perhaps Spock isn't trying to block the vent with his hands, but is actually trying to mind meld with the vent to get it to close? More likely he was hoping to mind meld with the creature.
  3. Spock says his hemoglobin is based on copper, so shouldn't it be called cupraglobin. He may prefer to use the more familiar term, especially if both substances perform the same function. Mike Konczewski on Monday, April 19, 1999 - 7:21 am: Keith - the prefix "hemo-" is from the Greek word for blood, not iron. Copper based hemoglobin is not a contradiction in terms.
  4. Spock says that it will take two men to transport the antimatter, but on the surface Garrovick lets go of it and the device hovers. The device is probably fitted with anti-gravs, which would reduce the number of people needed to carry/move it.
  5. I thought that antimatter could not be transported? I guess the crew was unaware of that fact and did it anyway. Scotty may have developed a way to transport the antimatter.
  6. All the security people in this episode seem to be Ensigns, but I believe both Yar and Worf of The Next Generation were Lieutenants. The security Lieutenants were probably left on Enterprise.

Internet Movie DatabaseEdit

Character errorEdit

  1. At the hearing, Ensign Garrovick describes the creature as being "several cubic meters" in size, but his distance as "about twenty feet away," mixing two different systems of measurement. It's unlikely that people in the Trek Universe would have any use for the antiquated, inefficient system of feet and miles. He was probably comparing the distance to the size of his feet.


  1. Kirk's hair is mussed from his fistfight with Ensign Garrovick, but appears freshly combed when they're beamed up immediately afterward. It doesn't look that different to me.
  2. Kirk and Garrovick are in clean clothes when they beam up, despite having gotten very dirty. The transporter biofilter may have an experimental dirt removal subroutine, added to the operating system after the events of The Enemy Within.

Plot holesEdit

  1. Lt. Leslie apparently comes back from the dead, due to an important plot element in the script that was never filmed (Although Lt. Leslie is killed by cloud creature, he is seen later in the episode walking in the background. According to Eddie Paskey, a scene was written in which Leslie and the others killed by creature are revived by Dr. McCoy but it was not filmed. ) Perhaps some of the victims only appeared to be dead.


  1. Listed under Factual Errors on the IMDB entry.
  2. Of course, that part of the episode was edited out for syndication. Does that mean that it never happened?


  1. Obsession at Wikipedia

The Original Series Season 2
Catspaw I Metamorphosis I Friday's Child I Who Mourns for Adonais?I Amok Time I The Doomsday Machine I Wolf in the Fold I The Changeling I The Apple I Mirror, Mirror I The Deadly Years I I, Mudd I The Trouble with Tribbles I Bread and Circuses I Journey to Babel I A Private Little War I The Gamesters of Triskelion I Obsession I The Immunity Syndrome I A Piece of the Action I By Any Other Name I Return to Tomorrow I Patterns of Force I The Ultimate Computer I The Omega Glory I Assignment: Earth