TOS - Arena (it was broadcast on the Heroes channel last week).
Funny, I'm pretty sure I've seen all TOS episodes multiple times, but sometimes I come across something that seems unfamiliar. I tuned in around 15 minutes late, and it took me most of Act 2 to figure out which episode it was (not until the conversation with the Metrons). Come to think of it, I still can't remember Act 1.
The thing that struck me at the end, when Kirk is magically transported back to the bridge after the fight, he is equally magically relieved of his leg injury and the dirt on his face!