The Enterprise visits a primitive world where the Klingon Empire has armed one faction of people against another in hopes of eliminating the weaker population and allying the stronger warriors with the Klingons. Spock is seriously injured when he, Kirk and McCoy beam down, and is returned to the Enterprise for treatment as Kirk and McCoy try to make contact with the locals. Kirk is injured by an poisonous indigenous animal, but reaches, with McCoy's help, his old friend Tyree. Tyree's mystical wife Nona cures Kirk and then pursues him. Kirk and McCoy, in the meantime, may only be able to resolve the unfair advantage between the planet's two factions by arming Tyree against his people's Klingon-backed adversaries.
