"Space...the final frontier." The iconic opening monologue of Star Trek: The Original Series perfectly set the stage for the challenges and adventures the crew of the Enterprise would face in that five-year mission. But just like with any frontier, the unknown scenarios would sometimes lead down some particularly dark paths.
Most of the time, the Enterprise's adventures would have a happy ending, even a joke or two. But that didn't negate some of the truly disturbing situations the crew members found themselves in while seeking out new forms of life and new civilizations.
Star Trek was never afraid of going dark if the story required it, and that darkness created some of the show's most memorable moments.
13. Triskelian Society In The Gamesters Of Triskelion
What started as a standard landing party for Kirk, Chekov, and Uhura quickly turned into a nightmare when the transporter beam is hijacked, carrying them to a completely different planet.
They are captured and forced to work as gladiators by the disembodied brains who run the planet Triskelion. Obedience is enforced through the Collars of Obedience, and every day they are forced to train and compete in battles to the death.
Of course, Kirk finds a way to get to the Providers and convince them it is better to help the thralls live in freedom and lead better lives for themselves than to keep them as slaves and sources of entertainment. But the dark lives of the thralls, the life that the landing party was subject to for a time, were not likely to change immediately.