During the episode, an alien female played by Marj Dusay beams aboard the Enterprise and, after incapacitating the rest of the crew, surgically removes Spock's brain.

Because of his unusual Vulcan physiology, Spock's body can be kept alive in this state, but for no more than twenty-four hours, giving Captain Kirk that much time to recover his stolen brain.

The Enterprise follows the alien ship's ion trail to the sixth planet of the Sigma Draconis system, a harsh world in the middle of an ice age.

Kara tells them that the skills needed to remove a brain were provided by a machine called the "Teacher", and that knowledge so obtained lasts no more than three hours.

Although the episode's story and teleplay were formally attributed under Writer's Guild of America West rules to former Star Trek producer Gene L. Coon (who specifically wrote its germinal pitch; initial story outline; and a sequence of first, second and revised second draft teleplays) under his "Lee Cronin" pseudonym (intended to telegraph his dissatisfaction with the final episode [while likely also attenuating potential career setbacks from his propinquity to the work] in a manner analogous to the Alan Smithee and Cordwainer Bird pseudonyms), incipient showrunner Fred Freiberger contributed a revised story outline in between Coon's first and second drafts that radically altered the gestation of the episode.

In Coon's April 1968 story outline, Spock's brain is taken by a group of Nefelese antagonists (led by a male named Ehr Von) during a survey of the surface of an asteroid.

Although McCoy studies advanced Nefelese medical techniques that ultimately enable the restoration of Spock's body (in tandem with his preexisting knowledge of Vulcan and human medicine), this is done without any human-machine interface.

Additionally, upon the completion of the surgery, Spock experiences side effects due to the reversal of several nerve endings, forcing him to laugh when he wants to sneeze; these complications (constituting a soupçon of Coon's characteristic levity) are ultimately surmounted through his mental discipline.

[2] William Shatner described it as one of the series' worst episodes, calling the plot a "tribute" to NBC executives who slashed the show's budget and placed it in a bad time slot.

[3][4] Leonard Nimoy wrote: "Frankly, during the entire shooting of that episode, I was embarrassed —a feeling that overcame me many times during the final season of Star Trek.

[7] A device similar to that used to remotely operate Spock's brainless body is used in the episode "The Magnificent Ferengi" of Star Trek: Deep Space Nine to animate a cadaver.

[21] The episode was released in Japan on December 21, 1993, as part of the complete season 3 LaserDisc set, Star Trek: Original Series log.3.