Written by George Clayton Johnson and directed by Marc Daniels, it featured design work by Wah Chang and first aired in the United States on September 8, 1966.

In the episode, the crew visit an outpost on planet M-113 to conduct routine medical exams on the residents using a Tricorder, only to be attacked by an alien that appears to change shape, and kills by extracting salt from the victim's body.

This was the first Star Trek episode to air on television, although the sixth to be filmed; it was chosen as the first of the series to be broadcast by the studio due to the horror plot.

After they stun him with a phaser beam, the dazed Crater reveals that the real Nancy was killed by the creature – the last member of a long-dead civilization of shape-shifters who feed on salt – a year earlier.

[1] Lee Erwin, who had previously worked on The Lieutenant with Roddenberry, was commissioned to produce a treatment; an outline featuring a salt-devouring vampire was handed in on April 8, 1966.

[3] Roddenberry felt that Johnson's treatment did not match his vision for Star Trek, but did not want to lose him entirely, and asked him to write "The Man Trap".

[4] Roddenberry and Justman made further tweaks; one was to restore the name "The Man Trap" from Erwin's original treatment,[8] another removed a scene which introduced McCoy's apprehension when using the transporter.

Michael Zaslow, seen in "The Man Trap" as Darnell, appeared in "I, Mudd", and later had a long-running role in the soap opera Guiding Light as Roger Thorpe.

Marc Daniels was recruited as director of "The Man Trap"; among his varied directing credits were episodes of I Love Lucy for Desilu Productions.

[19] During the production of "The Man Trap", Daniels introduced a system in which actors unneeded on a shoot went to a "cast table" area to practice upcoming scenes rather than return to their dressing rooms.

[26] Johnson praised both Chang and Gimpel, saying that while the latter embodied the character, the design work meant that, in the final scene, it was as if the crew had to kill a "helpless dog".

In his book The Influence of Star Trek on Television, Film and Culture, Lincoln Geraghty wrote that episodes such as "The Man Trap" demonstrated a recurring theme within the series: the more barren the planet, the more likely that characters will be in danger.

"[32] Geraghty also pointed out that elsewhere in the original series, the predominant view was that alien predators such as the salt vampire were a lower life form which should be destroyed.

[36] Block and Erdmann also discussed another part of the episode where Spock is in charge on the bridge and Uhura begins to flirt with him, calling it a "quintessential scene of the series" due to the characters' sexual interest in each other.

[38] A month prior to the premiere of Star Trek, Desilu held a screening for NBC executives to help decide which episode to broadcast first, and several stories were considered.

[39] Executives were concerned that "Mudd's Women", one potential choice, would have reviewers discussing "space hookers"; they felt another possibility, "Where No Man Has Gone Before", contained too much exposition, even though it was filmed as a second pilot.

[39] Justman felt that "The Naked Time" would make it easier for viewers to understand the characters, but later agreed with NBC's decision to show "The Man Trap" first.

[42] "The Man Trap" placed first in its timeslot, with Nielsen ratings of 25.2 during the first half-hour; some 46.7 percent of all American televisions in use at the time were tuned in to the episode.

During the second half hour, Star Trek was pushed into third with a rating of 12.3 by the Thursday night movie on CBS and the season premiere of Bewitched, which was also the first episode of that series broadcast in color.

[45] Canadian network CTV aired episodes of the first season of Star Trek on Tuesday nights instead of Thursdays and so ran "The Man Trap" on September 6, 1966, two days before NBC.

[47] A high-definition remastering of "The Man Trap", which introduced new special effects and starship exteriors as well as enhanced music and audio, was shown for the first time in the United States on September 29, 2007, in broadcast syndication.

Shiels preferred the also debuting Love on a Rooftop which, unlike Star Trek, provided background so viewers understood "what the show is supposed to be all about".

[51] In previewing the broadcast of "The Man Trap", The Daily Reporter said that Star Trek had the "usual far-fetched suppositions" present in other science fiction works, but praise was given to the acting skills of Shatner and the plots of the initial episodes.

[53] Daily Variety columnist Jack Hellman gave the episode an unfavorable review over its "lack of meaningful cast leads", who "move around with directorial precision with only violence to provide the excitement.

"[42] The weekly edition of the magazine offered a similar opinion, stating that the Enterprise "trudged along for a long hour with hardly any relief from violence, killing, ugly stuff and a distasteful monster".

[55] Ryan Britt, also writing for Tor.com, said that "The Man Trap" was not a good introduction to the series, but praised the screen time given to Rand, Uhura and Sulu.

[56] In Hollywood.com's ranking of all 79 episodes of The Original Series, Christian Blauvelt placed "The Man Trap" as 73rd, calling the creature "incredibly pointless".

[61] Star Trek news site Trek.Report gave the episode a ranking of 3 of 5, noting that actions of the crew were not affected by the "Prime Directive", a later addition to the franchise's canon.

[62] In 2024 Hollywood.com ranked The Man Trap at number 73 out of the 79 original series episodes, calling the concept of creatures that leach off of the salt in human bodies pointless.

[63] The first adaptation of "The Man Trap" was as a re-working into a short story by author James Blish as part of the novelization Star Trek.