A landing party from the Enterprise beams down to Cestus 3 for a dinner engagement with Commodore Traverse. Upon arrival, they discover that the Earth observation outpost has been obliterated. Furthermore, the outpost had been destroyed several days ago, so the messages received from the base must have been faked. The landing party (Kirk, Spock, Lang, Kelowitz, O'Herlihy, and Harold) discovers a survivor, but then comes under attack by the aliens responsible for the massacre. O'Herlihy is vaporized. The Enterprise simultaneously comes under attack by an alien vessel and is forced to raise its shields, preventing the landing party from beaming back up. Kirk is able to locate the arsenal of the outpost and locate a mortar. The aliens overload Spock's tricorder, causing it to explode, but Kirk is able to use the mortar to fire a blue sphere which destroys the alien attackers.
After returning to the ship and giving chase to the alien vessel, the survivor reports that they were attacked by a ship which approached at space normal speed, then proceeded to bombard the outpost. The Enterprise increases its speed to warp 8 in order to catch the alien. Before they can do so, both the Enterprise and the alien ship are scanned and incapacitated. The responsible party is an advanced civilization calling itself the Metrons and inhabiting an uncharted solar system. The Metrons force Kirk and the reptilian captain of the alien Gorn ship to duel to the death to determine which ship will survive. Both combatants are provided with auto-translation walkie-talkies, but Kirk thinks the device is only a recorder and so gives away his location and intentions to the Gorn. However, although the Gorn is physically stronger than Kirk, it is slower. Kirk rolls a (obviously styrofoam) boulder down a cliff and flattens the Gorn, only to see it push the rock aside and get up unscathed. Kirk then becomes ensnared in a set of ropes laid by the Gorn, but is able to escape, albeit with an injured leg.
After Spock's entreaties to the Metrons, the crew of the Enterprise is permitted to watch the spectacle on their video monitor. Kirk is eventually able to outwit the Gorn by finding the minerals for making gunpowder (sulfur, carbon, and potassium nitrate [saltpeter]) and using it to propel native diamonds (also found on this geologically unlikely planet) at the Gorn using a hollowed-out bamboo reed as the gun barrel. After winning a victory, Kirk convinces the Metrons not the destroy the Gorn ship, since it is possible that they viewed the Federation base as an encroachment on their territory. Kirk is returned to the Enterprise, which is the mysteriously transported back to its original position on the way to Cestus 3.