Day of the Dove
Distress signals attract Kirk and a Klingon commander to a desolate world. Kang takes over the Enterprise, but as violence and hatred spreads through both crews, he loses control.
Here is no doubt the most effective take on the "alien being feeding on emotion," since viewers feels the negative emotion flowing through all their normally considerate heroes. The Klingons, too, have some dignity at last, though as always we see them through the Starfleet officers' prejudiced gaze. The theme of overcoming fear and hate is not subtle, but it is effective, and less ham-fisted than the later Let That Be Your Last Battlefield. The swords are a good idea, especially how everyone gets a regional one, again distinguishing them by race. Scotty's "beauty" is most amusing. Chekov's imagined brother seems a bit of a stretch - is there no limit to what these blobs of light can do? - but it does bring out rage in a young officer who otherwise might not have any. The female Klingon is an interesting presence. The awkward-laugh ending at last has a point, as Kang and Kirk hope to rid the ship of the creature. We'd probably run away from this eerily giddy crew too.
Spock Saves the Day: Spock pieces together both crews' odd behavior into a reasonable theory, proven true by the emotion-fueled light alien.
Oops: A brief surge of racial hatred. Most distasteful.
Developments: The creature puts Spock back in "you don't wanna make me angry" mode, and he dangles Scotty by his neck! But in a few seconds the officers snap to their senses, and Spock realizes his actions were deplorable, apologing. And as the captains laugh the creature away, our Vulcan simply raises his eyesbrows, the most obvious display of levity he can allow himself.