TOS Season 1 Episode 18 Arena

Plot

The USS Enterprise arrives at the Cestus III Outpost by invitation of its commanding officer, but the crew find the outpost obliterated. Captain Kirk, First Officer Spock, Chief Medical Officer Dr. McCoy, and a security force beam down to find one survivor who says the base came under heavy bombardment from an unknown enemy. The landing party find themselves under fire from nearby, with two of the security team killed in the initial volley. The Enterprise is also under attack from an unknown vessel, preventing the crew from beaming up the landing party. On the surface, Kirk finds a grenade launcher from the outpost's stores, and uses it to scatter the alien forces. The alien ship recovers its crew from the surface and begins to retreat. The landing party is beamed back aboard Enterprise before they give chase.
Both ships enter an unexplored sector of space, and shortly thereafter, lose all propulsion power. Enterprise is contacted by a species calling themselves the Metrons (voiced by Vic Perrin), who zealously guard their sector of space from intrusion. They announce that they will pit the respective captains against each other in "trial by combat", a one-to-one battle to the death, with the ship of the losing captain to be destroyed and the other ship free to leave. Captain Kirk is suddenly transported to the surface of a rocky, barren planet along with the captain of the other ship, who is of a reptilian species known as the Gorn (voiced by Ted Cassidy). The Metrons speak to Kirk, explaining that while neither captain has communication with his ship, each has been given a vocal recording device that will transmit their words to their ships, however they are unaware that they also translate their words to the opposing captain as well as. Kirk is told that the planet has numerous resources either captain can use to defeat the other. Aboard Enterprise, the crew are allowed to watch Kirk's actions.
Kirk attempts to communicate with the Gorn, but receives no response. The Gorn tracks down Kirk, and Kirk realizes he is outmatched physically and relies on his agility to outrun the Gorn. Kirk gets caught in a rope trap set by the Gorn that injures his leg and slows him down. The Gorn finally communicates with Kirk via the translation device and offers to put him out of his misery. Kirk accuses the Gorns of being butchers, but the alien defends their attack on Cestus III, stating the outpost had been built in what the Gorns considered to be their territory. They viewed the Federation's presence in this part of space as an intrusion and a possible prelude to full-scale invasion.
Trying to stay ahead of the Gorn, Kirk discovers numerous valuable minerals and resources on the planet, seemingly useless at this point. He is inspired upon finding stalks of bamboo and raw chemicals that can be mixed into a black powder formula. He constructs a makeshift weapon, using chunks of diamond as ammunition. Kirk barely completes the assembly as the Gorn arrives and fires it, severely wounding the Gorn. As Kirk prepares to deal a death blow, he considers the Gorn's claims that the attack on Cestus III was only in self-defense, and allows him to live. Suddenly, the Gorn disappears, and a Metron (Carole Shelyne) appears to Kirk, congratulating him on not only winning the battle but showing the advanced trait of mercy for one's enemy. Kirk declines the Metrons' offer to destroy the Gorn ship, leading the Metron to comment that "you are still half savage, but there is hope", and that the Federation should seek out the Metrons again in several thousand years time. Suddenly Kirk finds himself back aboard Enterprise, his injuries healed, and the crew finds itself 500 parsecs from Metron space, the Gorn ship nowhere in range.