Filled with action, intrigue, a dash of horror and mystery, along with a good deal of fret by both sides of the coin, this episode brings the awful truth of wartime drama to the audience.
 This episode delivers a few memorable scenes of our heroic Enterprise officers behaving in atypical fashion, recalling a few other episodes where they were subverted mentally somehow. In this case, it involved reversion to basic primal instincts such as race hatred and bloodthirst, allowing actors Kelley, Doohan, Koenig and even the usually placid Nimoy to tap into their inner rage. The intense quarrel between Spock and Scotty is especially startling.
 Michael Ansara’s Kang was superbly cast in his role as the Klingon commander who has no qualms about torturing Chekov or shutting off life systems in those sections of the Enterprise which the Federation crew still control. Bixby also gives an important role to Mara, Kang’s wife and one of the only Klingon women ever depicted in TOS, as the peacemaker of the show who ultimately convinces Kang to reach a truce with Kirk.
-Excerpts from IMBD reviews
After receiving a distress call, the Enterprise goes to a Federation colony; when the landing party beams down, they find no one. Capt. Kirk (William Shatner) and the Enterprise have to deal w/ a nearby Klingon ship which they believe to be responsible. When the Klingon ship is disabled, they assume the attack came from the Enterprise. Commander Kang (Michael Ansara) argues with Kirk about who attacked who, then holds Kirk and his party hostage. Kirk sends Spock (Leonard Nimoy) a signal before they’re beamed up. Spock beams the landing party up and keeps the Klingons de-materialized until a security team is ready to subdue them. Kirk imprisons the Kilngons (about 40 in total). Kang wears the same golden sash that would be worn (on the opposite shoulder) by Lt. Worf in the first season of Star Trek: The Next Generation.
Spock informs Kirk that the Klingons were too far to attack the colony. After some Klingon plotting and waiting to take over, the Enterprise crew loses control (the ship starts warping in a random course). A large portion of the crew becomes trapped on an isolated area of the ship, leaving 40 men. The usual phasers we see turn into swords; this ep has sword fighting. Kirk uses a US Model 1860 cavalry saber. Scotty finds a type of sword in the armory which Scots are very proud of- a basket hilt Claymore (from the Elizabethan era or later).
Mr. Spock: [deflecting Scott’s maniac temper from Kirk] Easy, Mr. Scott.
Scott: Keep your Vulcan hands off me! Just keep away! Your feelings might be hurt, you green-blooded half-breed!
Mr. Spock: May I say that I have not thoroughly enjoyed serving with Humans? I find their illogic and foolish emotions a constant irritant.
Scott: Then transfer out, freak!
An alien creature of unknown origin has come aboard; it feeds off hate and violence! A wild-eyed Scotty (James Doohan) nearly gets into a fight w/ Spock (who is also changed) on the bridge; the captain has to intercede. Chekov (Walter Koenig) wants revenge for the death of a brother (though Sulu explains that he’s an only child). He nearly assaults Mara (Susan Howard), Kang’s wife/science officer, but is stopped by Kirk and Spock.
Dr. McCoy: Gentlemen, if we are pawns, you’re looking at one who is extremely sorry.
Mr. Spock: I understand, Doctor. I, too, felt a brief surge of racial bigotry. Most distasteful.
I was looking up reviews for this ep when I found a (hilarious) Trump parody.