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Capt. Kirk obsessively hunts for a mysterious cloud creature he encountered in his youth.


Ralph Senensky


Gene Roddenberry (created by), Art Wallace




Episode complete credited cast:
William Shatner ... Capt. Kirk
Leonard Nimoy ... Mr. Spock
DeForest Kelley ... Dr. McCoy
Stephen Brooks Stephen Brooks ... Ensign Garrovick
James Doohan ... Scott
Nichelle Nichols ... Uhura
Jerry Ayres Jerry Ayres ... Rizzo
Majel Barrett ... Nurse Chapel
Walter Koenig ... Chekov


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

Plot Summary | Plot Synopsis


TV-PG | See all certifications »

Did You Know?


The writer of this episode, Art Wallace, created and developed the story for the daytime soap opera, Dark Shadows (1966), which featured vampires, though Wallace was long gone from that series by the time they were introduced. See more »


When Kirk is re-materializing in the transporter after baiting the creature a black spot appears on his face momentarily during the process. See more »


Mr. Spock: You think you know what it was, Captain?
Capt. Kirk: Something that can't possibly exist, but it does.
See more »

Alternate Versions

Special Enhanced version Digitally Remastered with new exterior shots and remade opening theme song See more »


Referenced in Star Trek Continues: The White Iris (2015) See more »


Music credited to Alexander Courage, although it strongly resembles the main title music for 'Hollow Triumph (1948)' by Sol Kaplan
Sung by Loulie Jean Norman
See more »

User Reviews

Military Drama Posing as a Space Monster Adventure
11 October 2007 | by mstomasoSee all my reviews

Obsession features a great Art Wallace script and solid performances by Shatner, Kelley and Nimoy. An away team is investigating a seemingly uninhabited planet, while carrying a cargo of medical supplies to a rendezvous for which they can't be late. A mysterious gas cloud attacks and kills a trio of red shirts by consuming their red corpuscles. Kirk recognizes the smell (literally) of all of this and explains that the last time he encountered this creature, half of his ship's crew was killed. The son if the captain Kirk served with is now a member of the Enterprise crew (Stephen Brooks) and Kirk believes that his hesitation doomed his crew mates. Soon, Kirk's obsessive behavior regarding the gas cloud has the crew questioning his sanity.

Ralph Senensky's direction keeps this classic Shatner episode tight, exciting, and well shot. There are many memorable scenes and the script allows for some very good character development for the young ensign as well as the principal cast members. And unlike the standard Star Trek Space Monster story, this ends up being as much a human-military (naval) drama as it is an adventure tale.

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Greek | English

Release Date:

15 December 1967 (USA) See more »

Company Credits

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Technical Specs


Sound Mix:

Mono | DTS (re-mastered version)| Dolby Digital (re-mastered version)


Color | Color (Technicolor)

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