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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Release Date:

15 December 1967 (USA)  »

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Aspect Ratio:

1.33 : 1
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Did You Know?


The incident in Kirk's life was eleven years ago, which is how much older Kelly is than Shatner and Nimoy. See more »


As Kirk converses with Ensign Garrovick in the ship's corridor, in their respective close-shots the curvature of the wall behind Kirk matches that of the wall behind Garrovick (who is directly facing Kirk); the corridor curvatures should be opposite for each character. See more »


Mr. Spock: There are many aspects of human irrationality I do not yet comprehend. Obsession, for one - the persistent, single-minded fixation on one idea.
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Featured in Bring Back... Star Trek (2009) See more »

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User Reviews

Star Trek Horror
1 August 2006 | by (New York City) – 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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