Spock undergoes the Vulcan mating ceremony.
When Spock starts behaving oddly, Captain Kirk asks Dr. McCoy to examine the Vulcan. Spock explains that he has entered pon farr, the Vulcan mating cycle: if he does not return to Vulcan to take a wife, he will die. Disobeying direct orders from Starfleet, Kirk takes Spock to Vulcan, rather than proceeding to inauguration ceremonies on Altair VI. Kirk and McCoy accompany Spock to the surface, where they will bear witness to the koon-ut-kal-if-fee marriage ceremony, conducted by the esteemed political figure T'Pau.
T'Pring, Spock's betrothed since childhood, invokes her right to ritual combat, demanding that Spock fight a champion of her choosing—and she surprises everyone by selecting Kirk as her champion, rather than her companion, Stonn. The captain agrees, only to discover belatedly that the fight is to the death. McCoy, noting that Vulcan's thin atmosphere places Kirk at a disadvantage, injects him with a tri-ox compound. Spock, deep in the plak tow, or blood fever, apparently kills Kirk soon into the combat. His grief canceling out his mating urge, Spock returns to the Enterprise, only to find that Kirk is alive and well, thanks to McCoy, who injected the captain with a drug that would simulate death, rather than the tri-ox compound that he claimed he was administering.