When Spock begins to exhibit strange, erratic behavior, Kirk asks McCoy to conduct a medical examination. McCoy concludes that Spock is experiencing a potentially lethal internal distress.

Spock explains to them that Vulcans are married as children with the understanding that they will fulfill this commitment when they become adults. Spock has reached this time, the "pon farr," and if he doesn't get to Vulcan immediately to mate with his bride, T'Pring, he will die. Kirk jeopardizes his career by disobeying a direct order to the contrary from Starfleet, and proceeds with all possible speed to Vulcan. As Spock's friends, Kirk and McCoy are invited to witness the marriage ritual — the "Koon-ut-kal-if-fee." T'Pau, a highly respected member of the high-counsel, will conduct the ceremony.

Trouble starts when T'Pring announces she would rather marry Stonn, a full Vulcan. T'Pring evokes her right to have Spock fight for her. However, she chooses Kirk as her champion. Fearing his friend is too weak to fight, Kirk agrees. It is only then he is informed that it is to be a fight to the death.

The fight ensues and Spock quickly demonstrates physical superiority. McCoy objects to T'Pau that Kirk isn't used to the Vulcan atmosphere and climate. He asks to inject the captain with a tri-ox compound to compensate. T'Pau agrees and Kirk is given the injection.

During the fight, Spock kills Kirk and McCoy accompanies the captain's body back to the U.S.S. Enterprise. Spock, his mating urges curbed by the knowledge that Kirk, his friend and captain, is dead by his own hand, relinquishes T'Pring to Stonn. He solemnly returns to the starship. There he finds Kirk alive and well, having been injected not with tri-ox, but with a knock-out drug which simulated the signs of death. Overjoyed, Spock smiles and utters a delighted "Jim!" before returning to his controlled, logical self.