Chekov: There was persecution on Earth once. I remember reading about it in my history class.
Sulu: Yes, but it happened way back in the twentieth century. There’s no such primitive thinking today.
This is one of those eps that I’m sure many non-Trekkers (or Trekkies) have read of/heard about. On the way to a mission, The Enterprise comes across a shuttlecraft stolen from Starbase 4 by Lokai (Lou Antonio- part of the chain gang in Cool Hand Luke), a humanoid who is half black and half white. Soon his pursuer, Commissioner Bele (Frank Gorshin- best known as The Riddler on the ’60s Batman series), arrives onboard (from an invisible ship- one of the biggest budget cuts in TOS). Bele demands that Lokai be turned over for transport to Cheron (their home planet) where Lokai has been convicted as a terrorist.
Spock: [referring to Bele and Lokai] Fascinating. Two irrevocably hostile humanoids.
Scotty: Disgusting is what I call ’em.
Mr. Spock: That description is not scientifically accurate.
Scotty: Mr. Spock, the word “disgusting” describes exactly what I feel about those two.
Kirk: That’s enough for today. Those two are beginning to affect you.
Bele regards Lokai as of an inferior race and claims that Lokai’s people were destroying their civilization. Lokai contends that Bele’s people enslaved his people, but then we learn that Lokai’s people engaged in mass destruction. Bele believes he is right (pursuing justice). Their hate for each other puts our heroes in danger; Kirk tries to convince them to stop fighting. Both men have superpowers and this pursuit has lasted 50,000 years!
Spock: Change is the essential process of all existence
The screenplay was based on a story by Lee Cronin (the pseudonym of Gene L. Coon). He had left Paramount and was under contract with Universal, so he was not supposed to be working for Paramount. The original story didn’t depict the aliens w/ bi-colored skin; one was a devil w/ a tail and the other was an angel. Director Jud Taylor came up w/ the idea of bi-colored skin shortly before filming. The plot was a (obvious/heavy-handed to critics and modern viewers) indictment of the discrimination/prejudice in the late ’60s. MLK, Jr. had been assassinated less than a year earlier. This was a few years after the Watts Riots (LA) and the events dramatized in popular movies: Ghosts of Mississippi, Malcolm X, and Mississippi Burning.
 This episode does have the marvelous self-destruct sequence initiated by Kirk, in which Spock & Scotty join in to voice the self-destruct codes. This sequence manages to squeeze out every bit of suspense possible for such a televised few minutes…
 There are a few good lines such as the scene where Spock tells Bele that his planet was once a violent world which the Vulcans eventually resolved through logic and cool reasoning.
 All theories are suggested by Spock, incl. nature vs. nurture. Their hated has outlasted the population of their planet. The only writing flaw is their hatred spans thousands of years. Nobody lives than long, except the “Q” maybe! The stock footage used for the burning of the planet looks suspiciously like the burning of Atlanta from GWTW, don’t you think?
-Excerpts from IMDB reviews