Synopsis
The Enterprise is called to a far-flung colony on Cestus III. When they get there, the colony has already been destroyed. We learn that, in the future, mortar shells don't launch shrapnel. Also Starfleet seems to have continued that ludicrous US project to shoot nukes from artillery cannons. Happy Fourth of July everyone!
This episode is a two-for-one first contact. First we have the gorn, a race of people in clumsy green prosthetics, and second we have the metrons, a race of superpowered narcissists. The Enterprise chases the gorn (who destroyed the Federation colony) into metron space, and the metrons decide to have Kirk face the gorn captain in the most awkward one-on-one fight imaginable.
After Kirk and the gorn captain get tired of throwing big pieces of foam at each other, Kirk realizes the planet has all the materials to make a gun just sort of laying around. He picks up a section of bamboo; mixes sulfur, potassium nitrate, and carbon; puts chunks of diamonds into the barrel; and somehow or other gets a bit of fabric lit.
Standing over the diamond-impaled gorn captain, Kirk decides to spare him. The metrons teleport the gorn away, and one of them addresses Kirk. The metron says that humans are still half-savage, but maybe in a thousand years the metrons will have more to say to the Federation.
Given what the Federation looks like in a thousand years, I'm not so sure:
Commentary
This may be the most iconic original series episode. You've got everything: red-shirts being disintegrated, ridiculous alien costumes, a plot driven by escalating contrivance, and a deeply optimistic resolution.
Back in "Balance of Terror," Kirk made the right call: you absolutely cannot show the romulans weakness. You have to meet them toe to toe, eye for eye. They send one of yours to the hospital, you send one of theirs to the morgue.
He takes that same approach with the gorn, but—he was wrong. The gorn were defending their borders. The shoe gets put on the other foot in "Conspiracy" (TNG 1x25), and in that situation Picard has no trouble vaporizing aliens. Indeed, the Federation and Starfleet are the carrot and the stick, and when push comes to shove, Starfleet has little reservations beating the shit out of invaders.
Or maybe I should let Quark make my point for me (from "The Siege of AR-558," DS9 7x08):
"Let me tell you something about humans, nephew. They're a wonderful, friendly people—as long as their bellies are full and their holosuites are working. But take away their creature comforts... deprive them of food, sleep, sonic showers... put their lives in jeopardy over an extended period of time... and those same friendly, intelligent, wonderful people will become as nasty and violent as the most bloodthirsty klingon. You don't believe me? Look at those faces, look at their eyes..."
"Arena" lets us bask in the optimism, the potential for the future, even if we don't deserve to. After all, Kirk was fully prepared to blow the gorn out of the sky.
