"Errand of Mercy" blends a hint of mystery and stunning double-edged title into a topnotch story that ratchets tension all the way to its surprise end. Writer Gene Coon masterfully sketches characters and situations both familiar and unfamiliar into a coherent whole widely regarded an integral part of the spiritual spine of Star Trek. The impact of the plot differs substantially from its Cold War introduction yet ages well, except perhaps for the strange "phaser torpedo" special effect and premiere of Klingons in their simple swarthy makeup (and issuing an occupation proclamation in English!).

John Colicos as Kor reigns not only as military governor but also arguably as best. Klingon. ever. in TOS. His dripping disdain for life, sentimentality and mercy (the emotions of peace) set the behavioral tone for his species for several generations at least. (It's a shame he didn't get more episodes.) The Organians are perfectly cast, from John Abbott as the friendly but restrained Ayelborne to David Hillary Hughes as the "really quite intuitive" Trefayne announcing starships and soldiers in an eerie, subdued performance. Peter Brocco as Claymare doesn't say much but pulls the denouement together with his sincere emotional distress at the mere presence of "beings like yourselves."

The Organians still opt to interact with far more primitive species, like the Metrons but with a cool citadel. The primitive forms they assume reveal what they think of humanoids even discounting self-opening doors and ever-present "stupid idiotic smiles." They withhold their interference (no one dies, only stunned) until lethal violence is imminent and only then move to squash it completely. (Did they help Spock and the mind-sifter?) They appeared afterwards in several screen and novel references before being severely maligned in Enterprise.

As a ram among sheep Kirk is barely able to suppress his nature as soldier and not diplomat ("You do not like to be pushed") long enough for his secret to be nakedly spilled anyway. Spock fibs stupendously about his dealings in kevas and trillium, a practical lie that gains time but little else, and actually suggests violence as a "more direct approach." (Good to learn that the Vulcan nerve pinch works on Klingons also! Or at least augmented ones.) These acts erode the morality of their cause, in that "All we ask is that you let us help you" carries an enormous burden of previously unexamined (and misunderstood, as Ayelborne points out) arrogance. There is no peaceful way to defend the death of uncounted innocents and destruction of life on a planetary scale.