As cited in Star Trek FAQ: Everything Left to Know About the First Voyages of the Starship Enterprise by Mark Clark, DeForest Kelley's favorite episode of Star Trek is season 3, episode 12, "The Empath." An unconventional episode of Star Trek, "The Empath" has some great Dr. McCoy moments, but it features some disturbing torture scenes that resulted in the episode being banned.
The episode begins when the Starship Enterprise arrives at Minara II to find the research station there deserted. Captain Kirk, Spock, and Dr. McCoy are soon transported to a strange underground chamber where they encounter a woman with mutism whom McCoy dubs Gem (Kathryn Hays). From there, two alien Vians attack and torture Kirk and McCoy, and Gem uses her empathic abilities to heal them.
Why Star Trek's "The Empath" Was Banned In The UK
"The Empath" Did Not Air On The BBC Until 1994
Throughout "The Empath," Kirk, Spock, and McCoy discover that the Vians conducted horrific experiments on the personnel from the research station, ultimately leading to their deaths. The Vians proceed to torture Kirk and later, McCoy, in some of Star Trek: The Original Series' most disturbing scenes, leading to the episode being banned in the UK.
Regarding their decision not to screen "The Empath" and three other Star Trek: The Original Series episodes, the BBC cited the episodes' inclusion of "unpleasant" topics such as "madness, torture, sadism, and disease." Indeed, "The Empath" includes graphic (for the time) scenes of torture that remain difficult to watch even today.
"The Empath" Was Not The Only Star Trek Episode To Get Banned In The UK
The BBC Banned Four Total Episodes Of TOS Until The 1990s
After the BBC looked more closely into Star Trek, they ultimately banned four total episodes of the series, including "Miri," "The Empath," "Plato's Stepchildren," and "Whom Gods Destroy." Aside from citing that these episodes dealt with the unpleasant topics listed above, the BBC never provided any further reasoning for their decision.
The reasons for banning "Whom Gods Destroy" are less clear, as its depictions of torture are far less violent than those seen in other episodes that were not banned. Still, the BBC did not air any of these four episodes until the mid-1990s, when they finally relented and incorporated them back into reruns of the series.
Why "The Empath" Is A Memorable Star Trek Episode
"The Empath" Has Become A Particularly Polarizing Episode of TOS
With its mostly dark setting and simple plot, "The Empath" is an atypical and very polarizing episode of Star Trek: The Original Series. I've seen the episode pop up on lists of the worst Trek episodes ever, as well as lists of standout episodes in TOS season 3, but everyone seems to have an opinion on it.
While "The Empath" uses unique cinematography and has some good moments for the Kirk, Spock, and McCoy trio, it's not particularly enjoyable to watch the sadistic Vians torment Gem and the landing party. Plus, the Vians suffer no consequences for the torture and murder they have committed, and, in the end, it's not clear they ever achieve a true understanding of compassion and empathy.
Notably, the episode shows Dr. McCoy at his most heroic, as he sedates Kirk and Spock to prevent them from being tortured by the Vians, offering himself up instead. He even tries to prevent Gem from healing him, as he does not want her to endure his pain.
"The Empath" is a great episode for Dr. McCoy and is worth watching for its uniqueness, but it remains a particularly divisive episode of Star Trek: The Original Series.
Source: Star Trek FAQ: Everything Left to Know About the First Voyages of the Starship Enterprise by Mark Clark