"Arena" sets the standard for Trek action in a most memorable episode for even casual fans. Responding to a dinner invitation at the frontier base Cestus III, Kirk and Spock beam down to find the colony destroyed and themselves smack dab in the middle of an ambush. Unable to identify their assailants they repulse the attack using photon grenades (how cool is that?) driving the ambushers back to their ship. The attack on Cestus III dominates the first part of the story, highlighting the effectiveness of Kirk during a surprise attack.

A high speed space chase begins, eventually traveling through an unknown star system where both ships are brought to an immediate, involuntary halt. Enterprise is then contacted by the Metrons, inhabitants of the system who seek to "put a stop to" the unwanted incursion. They snatch both captains off their ships to be dropped on another planet, where they must kill the other to save themselves. It is only now that Kirk comes face to face with his Gorn attacker, and what an unforgettable face that is!

Simple in concept and savage in appearance, the Gorn resembles nothing so more than a bipedal giant killer lizard with glittering compound eyes and a mouth full of nasty sharp pointy teeth. As Kirk continually has to remind himself, this is an intelligent creature capable of space travel and for that matter sufficiently cunning to pull off as sophisticated a ruse as they did. Unable to defeat his opponent in brute force combat Kirk must recall his basic chemistry and beat the clock to build a diamond shooting cannon out of local materials. Successful at felling the Gorn and moving in for the kill, Kirk refuses to finish him off just to entertain the Metrons. They in turn are astonished at this act of mercy and spare both combatants.

TOS came from an era in which implacable foes made good television, dealing with an enemy that can't be bargained or reasoned with, that doesn't feel pity or remorse, and absolutely will not stop until you are dead. For all their virtues later (or earlier) captains could not talk or technobabble a Gorn to death, though Archer might have received a spectacular beating. (It is inexcusable negligence that only Series V hinted any more at this fascinating species, and that solely with a poor CGI attempt.) Blame for the Cestus III massacre is allayed as that the Federation were actually the "space invaders." Fortunately their representative is up to both challenge and mercy.