Kirk and crew are carrying an Earth ambassador to establish peaceful, diplomatic relations with a planet currently at war. The kicker: the war is a digital one that's been faught in their cyberspace for more than 500 years. When the virtual missiles "strike", the effected persons must report to disintegration chambers within 24 hours. This maintains human casualties, but allows the respective civilizations to be preserved.
Dramatically, this episode is structured beautifully. You've got the diplomat who can't see beyond the facade of the war mongers below. You've got the long-laid and ultimately misguided history of a civilazation just begging to be interveened, and you've got a Captain with a certain affinity for breaking a prime directive that hasn't been mentioned yet.
When questioned, the representative of this civilization defends their method of conflict by arguing they've done away with the destructive ugliness of war and left only the necessary defining elements that are in line with our barbaric nature. Kirk replies,
"Instinct can be fought. We're human beings with the blood of a million savage years on our hands, but we can stop it. We can admit that we're killers, but we're not going to kill today. That's all it takes."
There's that patented Trek optimism, rearing its head again. It's astonishing to see such a basic yet phenomenally awesome first step towards the maturation of mankind. To stand over the carnage of our birth and define ourselves against animal beginnings, toward an evolved and tempered existence. Kirk's monologue encapsulates the basic ideological hope that must be embraced if mankind is to make it to the future depicted in Star Trek.
We end on a delightful exchange between Kirk and Spock, who is criticizing the riskiness of the Captain's destruction of the war machine:
-"Sometimes a feeling, Mr. Spock, is all we humans have to go on."
-"Captain, you almost make me believe in luck."
-"Why, Mr. Spock, you almost make me believe in miracles."