(written from a Production point of view)
A survey of Argus X brings the Enterprise crew in confrontation with a vampiric cloud that killed a crew Kirk was on years ago, captained by the father of an ensign currently assigned to the ship.
Captain James Kirk, Spock, Ensign Rizzo, Leslie, and a security guard are carrying out a survey of Argus X, a planet rich in tritanium, a material twenty times (21.4 according to Spock) harder than diamond. While appraising the material, Captain Kirk notices a sweet odor, and orders Rizzo and the security guards to perform a sweep with their tricorders of the perimeter of the landing site, with special instructions to scan for dikironium, and to fire on any gaseous clouds they might encounter. He notifies chief engineer Scott, standing by on the ship, that he is conducting an investigation of the area, in spite of the fact that the USS Yorktown expects the Enterprise to rendezvous with them in eight hours time.
Rizzo and the two other security officers report in when they encounter an odd cloud, and are ordered to fire on it immediately. Contact is then lost with the team. When Kirk and Spock run over to them to investigate, they find Rizzo badly injured and the other two officers dead. Their skin is oddly pale and Kirk says that every red corpuscle has been drained from their bodies.
- "Captain's log, stardate 3619.2. With the mysterious death of two crewmen, all personnel on the planet have been evacuated back to the ship."
Rizzo is quickly returned to the ship for treatment. Chief Medical Officer McCoy reports that Rizzo remained unconscious for some time, following massive blood transfusions. His autopsy of the deceased crewmen confirms that all red blood cells have been drained from their bodies, without any marks, cuts, or incisions of any kind. Captain Kirk suggested that McCoy examine the record tapes of the USS Farragut, which listed casualties eleven years earlier from identical causes.
At this point Kirk decides to investigate the officers' deaths further, in spite of the fact that the USS Enterprise is scheduled to pick up highly perishable medical supplies from the Yorktown, supplies that are badly needed on the planet Theta VII.
He further requests that McCoy revive Ensign Rizzo for questioning, which he does with cordrazine. Although half-conscious, and unreliable in McCoy's medical opinion, Rizzo reports that he remembered a sickly sweet odor and that he felt an intelligence when attacked. Kirk then leaves sickbay, requesting McCoy's medical report as soon as possible.
Kirk meets with Spock on the bridge, where Lieutenant Uhura reports an urgent message from Starfleet which is promptly ignored by the captain. Because sensor scans for dikironium were negative, Spock hypothesizes that the creature might be able to change its molecular structure and thus avoid detection from the Enterprise's sensor scans.
Kirk then receives word from Uhura that Ensign Rizzo had died. Ensign Garrovick, the new security officer, arrives on the bridge. Kirk asks if Garrovick's father is who he thinks. The ensign confirms this and says he expects no special treatment; Kirk says he will get none.
Kirk then asks if Garrovick is interested in pursuing the creature that killed Rizzo. Garrovick is: he was close to the late ensign, having graduated from the Academy together with him.
Kirk, Garrovick, and a party of four armed security officers beam down to the planet to investigate the phenomenon that killed Rizzo, with their phasers set on disruptor effect. They split into two parties of three, one led by Kirk (with Swenson and Bardoli), the other by Garrovick. Kirk gives orders that the cloud was to be shot on sight. Garrovick's party almost immediately encounters the cloud. While approaching them, it appears to hover briefly; Garrovick hesitates for an instant before firing. By the time he shoots, the cloud is moving fast. It attacks the two crewmen accompanying him, killing one of them.
- "Captain's log, stardate 3619.6. One of the men in critical condition, the other is dead. And I… I am now even more convinced that this is not only an intelligent creature, but the same which decimated the crew of the USS Farragut eleven years ago in another part of the galaxy. Both Spock and McCoy are doubtful of this, and I sense they also doubt my decision to stay and fight the thing. Why am I keeping the ship here?"
When the landing party returns to the ship, Kirk is now convinced that not only was the creature intelligent, but it was also the same creature which attacked and decimated the crew of the Farragut eleven years previously.
Kirk, Spock, and McCoy meet with Garrovick in the ship's briefing room to hear his report. He details his experiences, including his hesitation before firing. As punishment, Captain Kirk has him relieved of all duties and confined to quarters, an action McCoy finds to be too harsh on the young man. Kirk dismisses McCoy's words and then directs Spock and the doctor to make any observations in official reports. He hastily leaves.
At this point, Chief Engineer Scott reports that the Enterprise will be ready to leave orbit in half an hour. Kirk replies that they would not be leaving orbit, whereupon Scott reminds Kirk of the urgent nature of their mission to Theta VII. Kirk dresses Scott down, complaining that he is "tired of my senior officers conspiring against me." When pressed, he admits that he should not have used the word "conspire". He also severely admonishes acting science officer Ensign Chekov, who reports that they have conducted full scanner probes twice on the planet. Kirk yells at him to continue running scans twenty times if necessary until the gaseous creature was found.
At this point, Spock approaches Doctor McCoy to consult on what he has observed to be a persistent, single-minded fixation on the creature on the part of Captain Kirk. Spock informs McCoy that eleven years earlier, then-Lieutenant Kirk, under the command of Captain Garrovick on the Farragut, had encountered a similar situation. Captain Garrovick was the father of the ensign of the same name.
- "Personal log, stardate 3620.7. Have I the right to jeopardize my crew, my ship for a feeling I can't even put into words? No man achieves Starfleet command without relying on intuition, but have I made a rational decision? Am I letting the horrors of the past distort my judgment of the present?"
Shortly thereafter, Doctor McCoy meets with Captain Kirk in his quarters to discuss his recent actions. He speaks of the terrible stress of a young officer in his first real danger, but when Kirk snaps that punishing Ensign Garrovick was his command decision, McCoy reveals that the "young officer" he referred to had actually served aboard the USS Farragut – Lieutenant James T. Kirk, assigned to phaser control. McCoy reminded him that, as a young man, he had hesitated before firing on the creature, causing him to miss it. Shortly thereafter, it had killed two hundred crewmen, including Captain Garrovick. McCoy suggests that Kirk's guilt was causing him to become obsessed, and that he was preparing a medical log entry on Kirk's emotional condition. Such a log requires a witness of command grade, so McCoy produces such a witness: First Officer Spock.
As per regulations, Spock and McCoy inquire about his recent command decisions. Kirk defends his position by stating that he had sensed that the creature was intelligent, and that he felt it was the same one that attacked them on Argus X. Since this indicated the creature was dangerous and capable of space travel, Kirk had decided to pursue it. At this point, McCoy decides to withhold his judgment on Kirk's emotional state.
Chekov reports that scanners detected the cloud was heading into space, so Kirk orders the Enterprise to follow it and has the ship set on red alert. The cloud's speed surpassed warp eight; if the ship traveled at that speed for a prolonged period, it would damage the engines. After a brief hesitation, Kirk decides to abandon pursuit and the Enterprise slows to warp six.
As he could not leave his cabin, Nurse Chapel brings Garrovick his dinner. She finds him lying on his bed consumed with guilt over his inability to fire his phaser at the creature in time. When he insists he's not hungry, she claims that McCoy had her bring a microtape containing a simple order: to eat. She threatens to feed him intravenously in sickbay if he does not; unknown to him, this was a ruse, as Chapel's tape contained "A Survey on Cygnian Respiratory Diseases". After she leaves, Garrovick angrily throws the cover to his plate across the room, causing his ventilation control to jam. A red alert is then declared over the ship's intercom by Kirk, with an order for all crew to get to battle stations. Garrovick leaves his room and reports to the bridge in violation of orders.
The red alert was called due to the fact that the creature had decided to turn toward the ship. With Garrovick watching, the captain orders phasers to fire on it; however, the phaser attack is completely ineffectual, as is a barrage of photon torpedoes. The creature then enters the ship through an impulse engine vent that had been left open for repair. It enters the ship's ventilation system, killing one crewman and severely injuring another in the process, and leaving the crew of the Enterprise with only two hours of remaining air supply.
Ship's senior officers Kirk, Spock, Scott, and McCoy meet in the briefing room to discuss the situation. McCoy feels, more than ever, that the situation was a result of Kirk's obsession, but Spock counters that the question of obsession is now an academic one, as the creature had attacked. McCoy is taken aback by the scientifically-inclined Vulcan's use of the word "creature," the same description as the "obsessed" Human, so Spock responds that this attack – it had significantly changed its course to face the ship – indicated to him that it was, in fact, intelligent. The consensus of the meeting is that radioactive waste should be flushed into the ventilation system to drive out the creature. After the doctor and engineer leave to return to their posts, Spock also reminds Kirk that, as phasers were ineffective, there was no basis for his self-recrimination due to his actions on the Farragut – though the captain responds that he was not the crew member who needed to hear that message.
Later, Spock visits Ensign Garrovick in his stateroom. He informs him that his hesitation was natural, which Garrovick doesn't want to hear, but they are soon interrupted by the sickly smell of the cloud creature, coming through the vent. Spock ejects Garrovick from his cabin, seals the door, and attempts to reverse the vent, which is jammed.
From outside Garrovick's cabin, Kirk orders that the pressure inside be reversed. Garrovick lets the captain know that Spock had saved his life, but when he claims that he was the one who should be dead, Spock emerges from Garrovick's quarters and informs Kirk that neither of them died, the reverse pressure was effective and the vent was closed. When a stunned Kirk asks Spock how he could have survived the encounter, McCoy jokes that his green blood must have left a bad taste in the creature's mouth, to which Spock informs the doctor that the sarcastic nature of his comments did not make them any less accurate. Due to his copper-based blood, he was not affected by the creature. Kirk enters the cabin and is about to report a strange sensation to Spock, when he is interrupted by Scott, who reports that the creature is moving out of the ship the way it came.
Soon after, Kirk meets with Garrovick, recalling that he had been on the bridge during the attack. The ensign apologizes for violating orders, but, after commending his dedication, Kirk instead reminds him that, having been at the captain's side, he would have seen first-hand that phasers were ineffectual against the creature – since his hesitation made, even in Kirk's words, "no difference," he could report for duty.
Sensors indicate the cloud is moving off at high warp speed. Kirk, however, believes the creature had in Garrovick's cabin communicated its intent to go home. Course is therefore set for the planet Tycho IV, where the Farragut had encountered the creature eleven years earlier. McCoy logs his objections to the trip, as the medical supplies being brought by the Yorktown are urgent – and perishable. However, Kirk overrules him, particularly in light of Spock's conclusion that the evidence indicates the creature is going home to reproduce by fission – and by the thousands. It must be stopped as soon as possible.
The officers agree to use antimatter to destroy the creature, in spite of the fact that a matter/antimatter blast would rip away half the planet's atmosphere and that transporters might not function in such an environment. They further agree to use hemoplasm to attract the creature. Spock volunteers to go down to the planet's surface since he was resistant to the creature' corpuscle-draining attacks. However, Kirk overrules him and instead transports down with Ensign Garrovick.
Kirk and Garrovick beam down to the surface of the planet with the hemoplasm and one ounce of antimatter, which has the explosive force of more than ten thousand cobalt bombs. Unfortunately, while the antimatter is being primed, the creature takes the hemoplasm. As a result, Captain Kirk decides that to use himself as bait. He orders Garrovick to return to the ship. Wrongly thinking Kirk expects to kill himself in the explosion, Garrovick attempts to overpower him and force him to return to the ship. At this point the creature approaches both men. Kirk calls the Enterprise and tells them the exact moment to remotely detonate the bomb and transport them away.
On the Enterprise, Spock has some difficulty transporting them aboard. First, the crew tries to reset the transporter, then they cross-circuit to "A". They then decide to cross-circuit to "B", which enables them to beam the landing party aboard. Once safely aboard, Garrovick joins Kirk to hear some tall tales of his father's adventures.
"I need your advice."
"Then I need a drink."
- - Spock and McCoy, as McCoy is surprised to learn that Spock needs his advice on something
"Monsters come in many forms. You know the greatest monster of them all, Jim? Guilt."
- - McCoy, on Kirk's obsession with the cloud creature
"Intuition, however illogical, Mister Spock, is recognized as a command prerogative."
- - Kirk, on explaining his reasons for hunting the cloud creature
"You know, self-pity's a terrible first course. Why don't you try the soup instead?"
- - Chapel, convincing Garrovick to eat his dinner
"A survey on Cygnian respiratory diseases? I thought you took Garrovick some food. What were you doing with this?"
- - McCoy and Chapel, after she tricked Garrovick into thinking she had McCoy's medical orders
"He saved my life, captain; I should be lying dead in there, not him."
"Fortunately, neither of us is dead, ensign."
- - Garrovick and Spock
"Mister Spock, please don't take this the wrong way, but … why aren't you dead?!"
"It's that green blood of his."
"My hemoglobin is based on copper, not iron."
"I'll bet he left a bad taste in the creature's mouth, too"
"Colloquially expressed. But essentially correct."
- - Kirk, McCoy, and Spock, after Spock is unharmed by the cloud creature
"The way that thing can move, captain. I wouldn't believe it."
- - Chekov, to Kirk on the cloud creature's warp velocity
"Crazy way to travel! Spreading a man's molecules all over the universe!"
- - McCoy, as Spock and Scott have difficulty in beaming up Kirk and Garrovick
"Captain, thank heaven!"
"Mister Scott, there was no deity involved. It was my cross-circuiting to B that recovered them."
"Well, then, thank pitchforks and pointed ears!"
- - Scott, Spock, and McCoy, after Kirk and Garrovick beam aboard safely
- Story idea "Space Moby Dick" by Gene Roddenberry: 22 April 1966
- Story outline "Obsession" by Art Wallace: 19 May 1967
- Revised story outline: 23 May 1967
- First draft teleplay: 17 July 1967
- Revised first draft: 10 August 1967
- Second draft teleplay: 29 August 1967
- Final draft teleplay by Gene L. Coon: 6 September 1967
- Revised final draft by Roddenberry: 4 October 1967
- Additional page revisions by John Meredyth Lucas: 5 October 1967, 6 October 1967, 11 October 1967, 12 October 1967
- Filmed: 9 October 1967 – 16 October 1967
- Day 1 – 9 October 1967, Monday – Desilu Stage 9: Int. Bridge
- Day 2 – 10 October 1967, Tuesday – Desilu Stage 9: Int. Bridge, Transporter room
- Day 3 – 11 October 1967, Wednesday – Desilu Stage 10: Ext. Tycho IV surface; Desilu Stage 9: Int. Kirk's quarters
- Day 4 – 12 October 1967, Thursday – Desilu Stage 10: Ext. Argus X surface
- Day 5 – 13 October 1967, Friday – Desilu Stage 9: Int. Briefing room, Sickbay, McCoy's office
- Day 6 – 16 October 1967, Monday – Desilu Stage 9: Int. McCoy's office, Corridors, Garrovick's quarters
- Original airdate: 15 December 1967
- First UK airdate: 13 July 1970
Story and production
- Like "The Doomsday Machine", the script for this episode borrowed a page from Herman Melville's classic Moby Dick. (Star Trek 30 Years)
- Director Ralph Senensky noted, "I realized from day one that it was a transferring of the Captain Ahab-Moby Dick battle from the ocean to outer space. But the script was more than the novel's struggle between a man and a big whale; it was a mystery story, if not a "whodunit", a "whatisit". And it was more beyond that; it was a deep penetration into Kirk's psyche, his inner struggle to overcome guilt for his actions in a past incident." 
- Vincent McEveety was originally hired to direct this episode, however, since the series' mid-season renewal by NBC came so late, McEveety couldn't be secured for "Obsession", and accepted another assignment. Ralph Senensky was hired as a replacement after he completed his work on "Bread and Circuses". (Inside Star Trek: The Real Story, )
- On Friday 13 October 1967, Senensky left the set at sundown, to observe the Jewish holiday of Yom Kippur. Producer John Meredyth Lucas took his place for those few hours, making this his first directorial contribution to the series.  Lucas later went on to direct three episodes, "The Ultimate Computer", "Elaan of Troyius", and "The Enterprise Incident".
- This episode reveals that phaser-twos have a disruptor setting, also referred to as setting "disruptor-B".
- This episode also reveals more about the biochemistry of Spock, which is based on copper, not iron, and found to be distasteful to the creature.
- This episode also reveals that an explosion of less than one ounce of antimatter has the force of 10,000 cobalt bombs, which is capable of ripping away half of a planet's atmosphere, and may interfere with a transporter beam.
- The episode featured the death of Eddie Paskey's character, Leslie, who was killed by the cloud creature, but reappears in several subsequent scenes throughout the rest of the series very much alive. According to Paskey, a scene in the script in which Leslie is revived by a miracle potion was never filmed.  Director Ralph Senensky also confirmed that he did not shoot the scene. 
- Jerry Ayres, whose character Rizzo was killed by the cloud creature, also played Ensign O'Herlihy in "Arena", another doomed red shirt character. He had his hair dyed lighter in this appearance, to lessen the resemblance.
- George Takei (Sulu) does not appear in this episode.
- McCoy's short-sleeved medical tunic bears an engineering insignia instead of the usual sciences division insignia for the entire episode.
- The dikironium cloud creature was created using a smoke-making machine, hidden behind the rocks on the planet sound stage set. (Star Trek: The Original Series 365)
- In an effect unique to the series, the phaser beam that Garrovick fires at the creature is outlined with a dark border.
- When both Garrovick and Kirk retreat to their quarters to brood, the walls are bathed in deep purple light, reflecting their somber moods.
- According to the short story "The Greater Good" contained in the anthology Shards and Shadows, the mirror universe James T. Kirk, who was then a lieutenant, served aboard the ISS Farragut in 2264. Captain Garrovick took great pleasure in humiliating him in front of the entire crew. In revenge, Kirk arranged for Garrovick and two hundred of his crew to be killed by a dikironium cloud creature. He later claimed that he slept sounder the night after Garrovick's death than he had in years.
- In the videogame "Star Trek: Judgment Rites", Commander Ellis blames Kirk for the death of one of the Redshirts in this episode, a good friend of his from Starfleet Academy, and accuses Kirk of being callous with the lives of those under his command. He gives the man's name as Ralph Garvin, supposedly one of the Redshirts killed in the attack that left Ensign Garrovick mortally wounded (though not named in the episode itself).
Aside from the standard CGI replacement footage of the Enterprise, this episode most notably featured new effects shots of the planets Argus X and Tycho IV, as well as revised footage of the dikironium cloud creature and the Enterprise's phaser fire into it while in space.
One piece of new footage was added to this episode, a shot of the crater left from the antimatter blast on Tycho IV, shown at the end of the episode. This replaced stock footage of the Enterprise traveling through space.
- The next remastered episode to air was "Mudd's Women".
Video and DVD releases
- Original US Betamax release: 1986
- UK VHS release (two-episode tapes, CIC Video): Volume 25, catalog number VHR 2360, 7 May 1990
- US VHS release: 29 July 1991
- UK re-release (three-episode tapes, CIC Video): Volume 2.6, 2 June 1997
- Original US DVD release (single-disc): Volume 24, 5 June 2001
- As part of the TOS Season 2 DVD collection
- As part of the TOS-R Season 2 DVD collection
Links and references
- James Doohan as Scott
- Nichelle Nichols as Uhura
- Jerry Ayres as Rizzo
- Majel Barrett as Nurse Chapel
- Walter Koenig as Chekov
- William Blackburn as Hadley
- John Blower as Swensen
- Frank da Vinci as Vinci
- Steve Hershon as operations officer
- Jeannie Malone as Yeoman
- Eddie Paskey as Leslie
- Basil Poledouris as Bardoli
- Unknown actors as
2257; ability; advice; AID cleanup; alert station; analyst; angle; answer; antigrav; antimatter unit (aka antimatter container); antimatter; Argus X; assumption; "at this point"; atmosphere (aka air); autopsy report; bait; battle; battle stations; bearing; behavior; blood; blood count; blood transfusion; body; "Bones"; bowl; bravery; briefing room; camouflage; career; casualty; cc; chance; chemical element (aka element); cleaning; cloud; cobalt bomb; colloquially expressed; color; command grade; commanding officer; composition; confined to quarters; consciousness; conspiring; contact; copper; cordrazine; course; crater; creature; critical condition; cubic meter; cut; danger; data; day; death; deck; deep space assignment; deflectors; degree: deity; detonator; diamond; dikironium; dikironium cloud creature; dinner; distance; dreaming; drill; drug (aka medicine); duty officer; ear; effect; elevation; emotion; emotional condition; energy; estimation; event; evidence; evil; expression; failure; Farragut, USS; Farragut causalities; Farragut's executive officer; Federation survey vessel; feeling; fission; flush; frequency; friend; friendship; "gang up on you"; Garrovick; "get to the point"; gold; graduation; gravitational field; green; green light; growing season; guilt; hatch; heaven; hemisphere; hemoglobin; hemoplasm; hereditary trait; high warp speed; home; honey; hour; Human; idea; imagination; impression; impulse engine; "in case"; "in fact"; incision; information; injury; intelligence; "in that case"; intravenous; intuition; irrationality; iron; ivory; judgment; kilometer; kindness; laboratory experiment; lead; lifeform; light; light year; location; log entry; logic; magnification; magnetic vacuum field; manual; matter; matter-antimatter blast; maximum orbit; maximum warp; medical alert; medical analysis; medical emergency; medical log; medical record library; medical stores; Milky Way Galaxy; mind; molecule (molecular structure); molecular shift; monster; monster hunt; mouth; name; negative pressure; obsession (aka fixation); odor (aka aroma); on report; "on the surface"; opinion; orbit; ounce; percent; perimeter; permission; phaser; phaser range; phaser station; phaser-two; photon torpedo; physical condition; pitchfork; plan; planet; planetary survey; plastic; pollen; prerogative; prescription; present condition; pressure; price tag; problem; propulsion; propulsive force; psychology; pulse; question; radioactive disposal vent; radioactive flushing; radioactive waste; range; reasoning; recitation; record tape/tape record; red; red alert; red corpuscle (aka red blood cell); rendezvous; rendezvous point; reply; report; reproduction; respiration rate; result; risk; round trip; scanner; scanner probe; scanner range; scanning report; search; second; security officer; self-pity; self-recrimination; senior officer; sensor; sensor scan; ship surgeon; shock wave; size; soup; space; space travel; spawn; species; specimen; speed; spread pattern; "stand by"; Starfleet; Starfleet Academy; Starfleet survey vessel; stress; subconscious; subject; subspace message; surface; "Survey on Cygnian Respiratory Diseases, A"; survivor; survivor guilt; tactical situation; tall story; taste; "thank you"; Theta VII; Theta VII colony; "(a) thing"; thousand; threat; time factor; time sync; trap; treatment; tricorder; tritanium; trophy; Tycho IV; Tycho system; unconsciousness; universe; vaccine; vein; vent; vent control; ventilating system (aka ventilation system); wall; weapon; witness; wood; word; yard; year; Yorktown, USS; Yorktown's ship surgeon
- "Obsession" at StarTrek.com, the official Star Trek website
- "Obsession" at Memory Beta, the wiki for licensed Star Trek works
- "Obsession" at Wikipedia
- "Obsession" at MissionLogPodcast.com, a Roddenberry Star Trek podcast
- Director Ralph Senensky blogs about shooting this episode at Ralph's Trek
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