The Enterprise pursues aliens responsible for destroying a starbase--the lizark-like Gorn, when Kirk and the Gorn captain are both transported to the surface of a planet to resolve their differences in personal combat. The aliens responsible for transporting them, the Metrons, will destroy the loser. When Kirk wins by creating gunpowder, he requests mercy for the Gorn, impressing the Metrons, who conjecture that Humanity may one day advance beyond savagery.

Fredric Brown is credited as the author of this episode, but in fact the story was written by Gene Coon (perhaps unconsciously inspired by Brown) and the credit was given when the similarities to Brown's story were noticed.