Written by prolific horror and science fiction author Richard Matheson, “The Enemy Within” is the first of several ‘evil duplicate’ stories in Star Trek and stands up as an early example of the series as a genre in its own right.
This episode was Matheson’s second major screenplay that featured a screaming Shatner; the first was The Twilight Zone's “Nightmare at 20,000 Feet,” which is widely considered to be among the very best of that series. Matheson was invited by Roddenberry and the team at Desilu to attend a studio screening of “Where No Man Has Gone Before,” where he watched the pilot with several other writers that would make an impact on the series, including “The Man Trap” scribe George Clayton Johnson and Paul Schneider, who'd write “Balance of Terror” and “The Squire of Gothos.”
In an interview, Matheson recounted, “I had just looked at Dr. Jekyll and Mr. Hyde and immediately saw the potential of using that transporter device for separating the two sides of a person’s character. Having an accident with that offered a good way to study the alternative personality. And it was part of my original concept that he needed that negative element in his personality in order to be a good captain. I think, probably, we’re all mixtures of good and bad. If any one of us was all good, we’d be boring. And leaders have to have that drive and that ambition.”
However, when the story outline arrived, Roddenberry had mixed feelings. He saw that it would be a great episode for Shatner, but that an attempted rape might be too much for NBC’s censors. Matheson saw it as anything but exploitative and trivial, insisting that the balance between brutality and intelligence was a key part of Kirk’s personality. In fact, setting aside the “evil” Kirk’s attack on Yeoman Rand, Matheson’s outline shows him as an outright brute, drunk and reckless.
Another thing missing from Matheson’s original outline was the stranded away team. While Roddenberry and associate producers John D.F. Black and Robert Justman all loved the core concept, they thought that it was a bit too lean for their show. Matheson wasn’t happy about their comments, feeling that the ship-side story was more than enough for an episode of TV, but when his first draft was delivered, it featured Sulu and the landing party stranded on Alfa 177.
“I was a little disappointed that Roddenberry built in a necessity to have a ‘B-story’ about the members of his crew stuck on the planet,” Matheson later said. “I can see why he did it, because ‘B-stories’ seemed to be a very regular occurrence in television in those days, and maybe still are.”
Roddenberry sent Matheson a list of changes he wanted made to the script, foremost being that Dr. McCoy had replaced Dr. Piper since the pilot. He included a copy of the revised writer’s bible for the show and told Matheson “You will find a cynical “H.L. Mencken” quality which will be most helpful in your script which does use the Doctor considerably.”
In addition, Roddenberry wrote that they should be careful when portraying the “Evil” Kirk as a brute, mentioning that if he’s more cunning, there’s a greater danger attached to him. He also mentioned that the removal of his base impulses should have an effect on the “Good” Kirk as well, leaving him unable to make the gut decisions that’d gotten him where he was.
This change, along with Roddenberry and team’s insistence that Spock and McCoy’s personalities should be a key part of the proceedings, was exactly what the screenplay needed to become as effective as it is. When combined with a slate of good performances, “The Enemy Within” stands out as an episode of Star Trek that improves with multiple viewings.
Even with her fantastic performance, Grace Lee Whitney walked away very unhappy with the episode, especially the last scene in which Spock says “The imposter had some very interesting qualities, wouldn’t you say, Yeoman?”
In her autobiography, The Longest Trek: My Tour Of The Galaxy, she wrote:
"I can’t imagine any more cruel and insensitive comment a man (or Vulcan) could make to a woman who has just been through a sexual assault! But then, some men really do think that women want to be raped. So the writer of the script (ostensibly Richard Matheson - although the line could have been added by Gene Roddenberry or an assistant scribe) gives us a leering Mr. Spock who suggests that Yeoman Rand enjoyed being raped and found the evil Kirk attractive!"
Whitney also recounts that she’d had difficulty getting into the moment when shooting the scene in which Rand confronts Kirk after the attack. It had been several days and bringing out so much emotion was proving difficult. To help her, Shatner slapped the young woman across the face and the cameras began rolling.
A Second Aside
Director Leo Penn saw an opportunity to play up a dramatic moment with Spock’s line “There’s only one conclusion—we have an imposter aboard.” Originally the penultimate scene in Act One, Penn moved it to immediately before the act break to give the line its proper “oomph.”