Arena

The Enterprise comes under attack by unknown aliens while investigating the near-destruction of the Cestus III colony. While chasing the aliens into unexplored space, both ships are captured by the powerful Metrons, who force Kirk and the alien captain (later identified as a member of the Gorn race) to trial by combat; the winner's vessel will be set free, while the loser's ship will be destroyed.

Summary from Wikipedia, CC BY-SA 4.0.