Metamorphosis
A strange energy pulls the Shuttlecraft Galileo to a world inhabited by only one man and his "Companion." Kirk must teach them both the value of love and sacrifice if his companions are to leave.
A little low-key, but still an touching tale of boundary-crossing love. The nod to Federation history is appreciated, and makes our officer's concern and awe more believable than just meeting some unknown dude. The Companion is a colorful creature, though its many powers are not fully explained. What's really remarkable is that Kirk spends the better half of the show reasoning with it rather than fighting it. (Though they did try to kill it first, it didn't work.) Also praiseworthy is the captain's endorsement of interspecies love, which Cochrane finds "disgusting" at first. Inspector Hetford is just annoying, and it might have been better to feature a dying character to which we can attach some sympathy. The Companion's inability (refusal?) to heal her is, considering its powers, a plot hole. However contrived the means of getting them together, one can't help but smile at the happy couple in the end, and be moved by the sacrifices they make to stay together and understand each other. The scarf effect (with the now-human Companion trying to see "the man" as she always has) is the perfect touch.
Spock Saves the Day: Spock discovers the Companion's electrical nature and builds a device to combat it, then takes a more diplomatic approach and adjusts a universal translator.
Oops: Word up to Spock- when you encounter a seemingly hostile being made of pure electricity, for heaven's sake, don't touch it.
Developments: Spock must reminded Kirk he's supposedly a diplomat and shouldn't try to kill every creature in his path. Communication is still something Spock greatly values. He's also open-minded when it comes to love, likely because he is the product of an interspecies relationship.