This is top notch Trek, this is.
I found this episode inspired a slew of reactions. I was terrified, concerned, troubled, and moved. This episode deals with the the abuse of a man's status as a medical practitioner. The Enterprise crew clues into what's discovered to be the corrupt use of a mind-control device (is there any other?) on their visit to a penal colony.
An inmate (and former doctor, we find) escapes from the colony and makes his way onto the Enterprise, thus engaging our good Captain in the scandal. McCoy smells foul-play from the very beginning, after observing this patient's "condition." Kirk initially objects to McCoy's suspicions about the penal colony warden, Dr. Adams. In whats seems to be a plucky jab, McCoy assigns Dr. Helen Noel, with whom he has a history, to assist Kirk on the mission.
The immediate recognition and concern for his reputation Helen inspires in Kirk is very telling. It really helps to fill in a bit of his past, and cement the viewer's suspicions about Kirk's affinity for women. Obviously, in the course of the show (and pop culture) this becomes obvious. At this point in the program, however, this seems to be the first flagrant self-recognition.
Upon meeting the esteemed Dr. Adams, Helen is naturally impressed and rejects any sort of accusations. Kirk, thankfully, keeps his wits about him, and doesn't allow the charm of Dr. Adams to effect himself too much. Once the legitimate foul play is stumbled upon, Helen is forced to face the truth and act accordingly. If anything, I wouldn't have minded seeing this change of character expounded upon further. However, taking into consideration that this is an hour-long adventure series, I cannot blame them.
So Dr. Adam's mind control device was used on this poor escapee's brain, he's all fried and freaking out, so McCoy and Spock have to take care of him while Kirk investigates. It's a beautifully dramatic and heart-wrenching ordeal, watching this man suffer incalculable brain loss. The situation does birth, however, the first instance of the Vulcan Mind Meld. It's an intriguing sequence as Spock performs this alien ritual, calming the berserk victim.
In order to test his suspicions first hand, Kirk subjects himself to the machine secretly, with Helen at the controls. What ensues is a fairly humorous sequence in which Helen amps up the sauciness of Kirk's memory of their Christmas party encounter. In a bold and funny move, we actually see the newly created memory play out. What starts off as innocent quickly turns sinister and unfortunate as Dr. Adams interrupts. He then twists Kirk's mind to make him believe he's desperately in love with Helen.
Decent Shatner performance in these mind-chamber sequences, but the real kicker is the plot structure. Placing the tension of this action against the McCoy/Spock struggles on the Enterprise is very effective. Add to that the fact that the situation has escalated beyond pleasantries or facades, and you've got high drama: they're captives now. There's a sweet little moment where Helen could have very well taken advantage of Kirk's new and false infatuation with her, but she rises to the occasion and struggles to help him remember. Good stuff.
Eventually they create an escape plan, just as Spock has discovered the truth thanks to his Mind Meld. The episode soars in a final raid by Enterprise security against the penal colony personnel and Spock walking in on Kirk and Helen mid-kiss, making for a memorable end to a satisfying episode.
Well written and well-executed, this is a fine example of why Star Trek became the iconic success it is today.