Metron
Carole Shelyne

Lt. O'Herlihy
Jerry Ayres

Lt. Commander Kelowitz
Grant Woods

Lt. Harold
Tom Troupe

Lt. Lang
James Farley

Lt. DePaul
Sean Kenney

Voice of the Metrons
Vic Perrin

An emergency call summons the Enterprise to Cestus III but, by the time they arrive, the colony has already been destroyed. Shortly after their arrival both the landing party and the ship find themselves under attack from an unidentified race. Kirk, on the planet's surface, gives orders for the Enterprise to leave the area while he defends the ground position.

Finding a powerful mortar-like weapon in the colony's arsenal the landing party manages to fight off their attackers after which a mysterious, alien craft leaves orbit. They come across a single survivor from the original attack who tells them that the onslaught was unprovoked and that it was merely a means of attracting the Enterprise into a trap. Kirk believes that this could be a 'test run' for an alien invasion and, beaming back to the starship on its return, starts to pursue the alien vessel which is heading into an uncharted region.

After a long pursuit both ships are suddenly instantly stopped in space and a voice, announcing itself as belonging to the Metrons, tells them that it plans to resolve the situation. The Metrons transport both Kirk and the captain of the alien ship, a lizard-like creature called a Gorn, down to the surface of a planet in an uncharted system and tell them that there are sufficient materials there with which to construct weapons.

They will be fighting for their lives as the Metrons announce their intention of destroying the loser's ship. The Gorn is much physically stronger than Kirk and he has to use his wits and greater speed to survive. The crew of the Enterprise watches the trial by combat on their viewscreens as Kirk tries desperately to find the promised weapon materials. He digs deep into his memory for ideas and eventually gathers enough items to make a crude type of gunpowder.

Finding a large hollow tube he constructs a primitive cannon, using a large raw diamond as the projectile. The Gorn is lured into position and Kirk fires, severely disabling the alien creature. With the Gorn at his mercy he calls to the Metrons and tells them that he refuses to take the life of the alien.

At that point one of the Metrons appears and tells him that they had not expected him to display the advanced trait of mercy and that they had reconsidered their plan, which was to destroy the winner, not the loser as they had stated. The Enterprise is spared and the Metrons leave Kirk with the hope that, sometime in the distant future, their two civilisations may become friends.