The U.S.S. Enterprise is in pursuit of an unknown alien ship which has destroyed a Starfleet base on Cestus III. In an uncharted area of space, both the alien ship and the Enterprise are caught by an advanced race called Metrons. The Metrons are angry at the two ships for trespassing into their space and believe that physical combat is the answer to finding justice.
They transport Kirk and the alien captain, a lizard-like creature called a Gorn, to an uninhabited asteroid to fight to the death. The Metrons promise that the victor and his ship will be set free, while the loser will be destroyed, along with his ship and crew.
The Gorn informs Kirk that the base on Cestus III was destroyed because it was believed to be a hostile intrusion on Gorn space. The Gorn is seven feet tall and much stronger than a human, with an extremely aggressive nature, but Kirk has the advantage of speed and agility that his opponent lacks. Kirk manages to keep out of the Gorn's way long enough to mix local minerals into gunpowder, which he uses in a primitive cannon to wound the Gorn captain.
When Kirk refuses to kill the Gorn, the Metrons decide that there may be some hope for the human species after all. Both captains and their ships are set free.