1.18 Arena

When the Enterprise is locked in combat with the aggressive Gorns in space, their battle trespasses into the territory of the advanced Metrons who decree a trial of combat between Captain Kirk and the Gorn Captain!

Plot Summary
Stardate: 3045.6 - 3046.2 [1]First use of the photon torpedoesThe Enterprise pursues an alien ship after it destroys a Starfleet base on Cestus III. While in pursuit, both ships are halted by an advance race called the Metrons, who are displeased that the two ships are fighting inside the race's territory. The Metrons teleport Kirk and the alien captain, who they learn is called a Gorn, to the surface of an asteroid. They command Kirk and the Gorn captain to fight to the death. The winner is returned to their ship and allowed to leave safely while the loser's ship is destroyed. As Kirk battles the Gorn, he quickly learns that the Gorn is superior to him in strength and endurance, but lacks his agility and speed. As they battle, the Gorn captain explains that the Gorn had destroyed the Starfleet base because it was an intrusion on their space territory. Kirk continues to run away from the Gorn, but he knows that his stamina is running out. He soon figures out that he can use the minerals on the asteroid to form gunpowder. [2]Kirk sparing the GornKirk makes gunpowder and uses the diamonds he finds to make a makeshift cannon. He fires the cannon at the Gorn captain and stands poised to give the finishing blow. However, he decides to spare the Gorn. A Metron appears and is pleased by what he sees. He tells Kirk that there may be hope in humanity yet. He transports both Kirk and the Gorn captain back to their ships and allows them both to leave safely.

Production Notes
The rubber costume was designed by Wah Chang who had designed many of Star Trek's props and costumes. Chang had designed many of Star Trek's iconic props such as the tricorder, communicator, and the first Romulan ship. In this episode, Chang was reportedly pressed for time and had lacked the funds to create a more detailed costume. What came from his efforts was the infamous Gorn costume. In the next appearance of the Gorn, the alien would be created using computer animation.