Sent to the planet Capella IV to negotiate a mining treaty, Kirk and a party beam to the surface. They find the Capellans to be warlike and tradition-bound people. Kirk also finds that a Klingon agent, Kras, has gotten there before them and established an agreement with some of the planet's rebels who kill their leader, Akaar, and take over. Akaar's widow, Eleen, is willing to forfeit her life, as custom demands, because she carries the child that will be the next leader, or Teer.
Kirk convinces her to escape and they hide in the hills beyond the city. While the landing party evades pursuit, a Klingon warbird prevents the U.S.S. Enterprise from helping its people.
When Eleen goes into labor, McCoy delivers the child. Eleen, however, hits the doctor over the head with a rock, knocking him out, and returns to the Capellans. She tells them that the landing party and the child are dead. The Klingon decides this is the time to take control and begins to attack the Capellans.
Arriving on the scene, Kirk and Spock try to use primitive bows and arrows on the Klingons which wound, but do not stop. Maab, the new Teer, draws the Klingon fire while his lieutenant kills him. Eleen names her son Leonard James Akaar, after Kirk and McCoy, and as her son's regent until he comes of age, signs the mining treaty with the Federation.