Was it the one where Kirk used "half-breed" as an insult so horrible and hurtful that it would reveal an android as an imposter (in a period where segregation was still legal in the US)?
Was it the one about a long-standing war between two peoples based entirely on the color of their skin?
How about the one with the Nazi planet?
Surely you're not referring to the one with the first interracial kiss - something so political that the network debated censoring it and caused Shatner to intentionally ruin takes of an alternative version?
Are you talking about the one which involved Kirk explicitly promoting contraception and calls out the hypocrisy of the self-proclaimed "pro-life" alien race?
No, perhaps you're thinking of the one involving the torture of the leader of a revolutionary group.