Star Trek (1966–1969)
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Capt. Kirk obsessively hunts for a mysterious cloud creature he encountered in his youth.



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Episode complete credited cast:
Stephen Brooks ...
Jerry Ayres ...


Captain Kirk is haunted by a creature from his past when conducting a mining survey on a planet. He first encountered it as a lieutenant aboard the U.S.S. Farragut and blames himself for freezing in a moment of crisis, causing the death of many crewmen. The creature is a cloud-like, gaseous being that lives on the red blood cells found in humans. Obsessed by his desire for revenge and to erase the memory of 11 years ago, he pursues the creature relentlessly, putting in jeopardy an assignment to collect essential medical supplies. Written by garykmcd

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TV-PG | See all certifications »


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15 December 1967 (USA)  »

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Did You Know?


In the medical records library between Spock and McCoy, there is a large object which looks like a vertical DVD rack. The same large object was in Harry Mudd's laboratory in Star Trek: I, Mudd (1967). See more »


Ensign Garrovick states that less than one ounce of antimatter is more powerful than 10,000 cobalt bombs but the actual energy yield is only about 1.2 megatons TNT. The implication that a cobalt bomb releases more energy than a "regular" nuclear weapon is incorrect; a cobalt bomb is just an ordinary thermonuclear weapon "salted" with cobalt to produce long-lived Co-60 in its fallout. See more »


Mr. Spock: You think you know what it was, Captain?
Capt. Kirk: Something that can't possibly exist, but it does.
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Referenced in Star Trek Continues: The White Iris (2015) See more »

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User Reviews

Star Trek Horror
1 August 2006 | by See all my reviews

Obsession offers a very extraordinary antagonist--a "monster" that's an intelligent gas, made of an element that's not supposed to exist "naturally", and that's able to transform itself as a means of camouflage. Although there are some very broad parallels to this idea in other episodes, including Who Mourns for Adonais?, the material is handled very uniquely here.

We are also presented with an intriguing and unusual exploration of Captain Kirk's psychology, and through that, a bit of Starfleet history. Kirk is the source of the title, Obsession. He harbors deeply buried guilt, stemming from an incident on another ship when he was still a junior officer. His guilt suddenly rises to the surface after encountering this episode's antagonist. Because of this, he becomes obsessed with conquering the "monster", and it affects his judgments seriously enough that his command of the Enterprise is threatened.

As good as it was, the original Star Trek didn't often get close to more gut-level, scary horror material. This episode does, largely because the antagonist is unpredictable, shape shifting and nearly invisible much of the time. It functions as more of a generalized token of a threat to one's life force, made more literal in the way that the "monster" attacks its victims and sustains itself. At times, Obsession's structure and content--including emotional content, almost resemble later horror sci-fi like Alien (1979), which could have easily been influenced by it.

This episode is also notable for some extended, unusual cooperation and agreement between Spock and McCoy.

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