Taking its title from a Charles Dickens quote, this Kirk/Spock episode takes its cue from the Cold War and the Cuban Missile Crisis, developing a "battle of the super powers" story before ending it all with a bizarre science fiction twist.
Just as 20th Century America is easier to define by contrasting it with the Soviet Union, Gene Coon better defines Kirk and the Federation by creating a worthy adversary, the Klingons, with John Colicos stepping into the shoes of their leader, General Kor. Named after Officer Wilbur Clingan (1920-2012), who served with Gene Roddenberry in the Los Angeles Police Department, the new alien enemies literally enter the Star Trek universe with guns a blazing and press their advantage throughout the episode, forcing Kirk and Spock (and Sulu on the Enterprise) to play defense. (Curiously, Dr. McCoy does not appear in the episode at all). A planet-based story, Organia is brought to life by the Arab village portion of the Desilu Culver backlot, offering Kirk and Kor a realistic environment to spar. Colicos plays it to the hilt, all but twirling his evil mustache and setting the standard for all Klingon leaders to come. (He also seemingly invents texting, using his fingers to send a message through is communicator late in the episode. Or maybe his character is scrolling through downloaded information. Either way it's quite a thing to see in a 1960s TV show!)
Meanwhile, everyone overlooks the Organians (not to be confused with the Oregonians), the peaceful aliens caught in the middle. English Shakespearian actor John Abbott deserves special praise for his seemingly simple portrayal of their Ghandi-like leader, using a disconcerting lack of subtext clues to hide the complexity of his performance. In the end, the episode's great irony is how his character forces Kirk and Kor, who have more in common with each other than the planet's inhabitants, to actually develop a respect for each other borne out of their frustration for the locals. (Meanwhile, the Organian leader mentions offhand how the Klingon Empire and the Federation will eventually become friends and work together, which sets up TNG and Star Trek VI some twenty five years ahead of time, as if the franchise had it all worked out from the beginning). It all leads to Kirk being wrong again and learning another lesson, with Star Trek making an ambitious anti-war statement for 1967. This time, however, Spock points out that Kirk has nothing to be ashamed of, which is not only a good message for the good Captain but also for those of us rooting for him.
Sadly, Kor never returns in the original series. He does, however, appear in the animated series episode "The Time Trap" (voiced by James Doohan) and (reprised by Colicos) in three Deep Space Nine episodes, beginning with "Blood Oath".
The Organians finally return to the Star Trek universe in a prequel of sorts to "Errand of Mercy" in Enterprise's fourth season episode, "Observer Effect".
Remastered: With the Klingons pounding the Enterprise in the ship-based sub-story, you'd think there was a lot for CBS Digital to do, but the truth is the writers intentionally keep most of the action on the planet to avoid expensive effect shots. (Indeed, the episode only has a few shots of the Enterprise actually getting hit, borrowing shots of the Enterprise being fired on from "Balance of Terror" and "Arena" to fill things out and artfully avoiding having to show the Klingon ships). The upgraded version does have some nice battle footage, including debris, and even shows the Klingon fleet; but the shots are quick. Down on the planet, most of the footage is as it always was. (Anyone who thought they'd rotoscope in big Klingon foreheads to match the Klingon look in the films, which would probably cost $100 million and take ten years per episode, is out of their gourd). They do touch up the Organian transformation to make it a little less cheesy. They do not, however, change the stock footage used in the original, continuing to use an old shot of Haiti's Citadelle Laferrire as the makeshift Klingon headquarters. Originally this was supposed to be a matte painting, but Roddenberry simply couldn't afford another one. It's too bad the same thing happens again with CBS Digital, because it's obviously a stock shot, and replacing it with a new digital matte would be a big improvement.