Stardate 2715.1 through 2715.2
Approximately 2266 AD
Approaching the planet Tantalus V, home of the Tantalus Penal Colony where the famous Dr. Tristan Adams treats his mentally ill and violent criminals, Enterprise and crew receive a package from the surface. Stowawayed inside the package is the escaped Dr. Simon Van Gelder who has gone violently insane under mysterious circumstances. The crew manage to restrain the renegade doctor but Gelder continues to rave about the dangers of returning to Tantalus and how Dr. Adams is not to be trusted. The only people who believe something is wrong on the surface is Dr. Bones and, surprisingly, Spock.
Kirk decides to go to the surface to investigate and he brings along the ship's psychologist, Dr. Helen Noel. Upon his investigation Kirk discovers a machine that is capable of making it's victims forget their past experiences and allows the user to suggest false memories. We quickly learn that for all the good Dr. Adams has done for the criminally insane, he has lost touch with his ethics and began using the machine against the will of his patients
This might be the first episode of The Original Series that is void of silly dialogue, poorly disguised aliens, or some supernatural race. This episode is a pure human drama that explores the consequences of not only experimentation on humans, but also of the need of past experiences to define our personhood. The machine essentially makes the human mind completely blank. The episode suggests that when the mind is blank it becomes like a sponge that accepts any suggestion given to it because of the emptiness and corresponding loneliness. So when Kirk is subjected to the predations of the machine he understands deeply the pain that is felt by Gelder, musing, "Can you imagine a mind emptied by that thing, without even a tormentor for company?"
The sympathy that Kirk has towards those who went under the machine is actually portrayed very lifelike by William Shatner. At the end of the episode we can see Kirk's understanding of the eternal horrors through Shatner's face. DeForest Kelley, too, did a fantastic job as Dr. McCoy in this episode. As a doctor he didn't immediately dismiss the ravings of Gelder but rather showed genuine sympathy towards this character. This sympathy is ultimately the driving force behind Kirk investigating the colony in the first place.
These kinds of episodes are why I enjoy Star Trek so much. Not the aliens or ponderings of what the future will look like, but rather the ethical questions that are oftentimes asked. Dr. Adams seemed to believe that if you empty the human mind of it's cold and darkness, then there will be room to fill it with love and warmth. But in the pursuit of this endeaver Adams created a torture for those who expereiced the machine, literally killing them from loneliness. Dr. McCoy said that it is hard to believe that one can possibly die from loneliness, Kirk responds "not when you've sat in that room."
