Spock requests a leave of absence to his home planet of Vulcan after displaying irrational behavior. Captain Kirk, commander of the Federation starship Enterprise, and Chief Medical Officer Dr. McCoy witness one of Spock's outbursts, and McCoy agrees Spock needs some "time off." Kirk is baffled by Spock's behavior, but diverts the Enterprise to Vulcan. Soon, however, Kirk receives a priority signal with orders to proceed to Altair VI to represent the Federation at an inauguration ceremony for the planet's new president. Kirk tells Spock his leave will have to be delayed, but Spock secretly orders the ship rerouted to Vulcan. Kirk confronts Spock, who says he has no memory of changing Kirk's order. Kirk directs him to report to Sickbay. Dr. McCoy discovers Spock's blood chemistry is extremely active and has the presence of unknown hormones. If the condition persists, Spock will die in eight days. Spock does not wish to discuss what is currently affecting him, but Kirk demands an explanation. Seemingly embarrassed, Spock informs Kirk that his condition is called pon farr, a syndrome that all Vulcan males painfully endure periodically throughout their adult life.[note 1] During this time, they must mate or die. He cites precedents in nature, such as Earth salmon, which "must return to that one stream where they were born, to spawn … or die trying."
Kirk contacts Admiral Komack at Starfleet and requests permission to divert to Vulcan. The admiral denies permission, but Kirk ignores the order, arguing there are already two other starships attending the inauguration. The Enterprise arrives at Vulcan, and Spock invites Kirk and Dr. McCoy to accompany him. Spock explains that Vulcans are bonded as children ("less than marriage, more than a betrothal") with the understanding that they will fulfill this commitment when they become adults. His bride T'Pring, who was betrothed to him at the age of 7 (played by Mary Elizabeth Rice in an image on a view-screen in Spock's quarters), awaits him.
T'Pau, a highly respected member of Vulcan society, best known elsewhere as the only person to ever turn down a seat on the Federation Council, arrives to conduct the ceremony. T'Pring arrives accompanied by Stonn, a pure-blooded Vulcan, who is obviously her lover. She invokes koon-ut-kal-if-fee, her right to a physical challenge between Spock and Stonn, but instead of Stonn, she picks Kirk to be her champion. Spock asks that T'Pau forbid it, because Kirk "does not understand, he does not know," but T'Pau allows Kirk to decide, telling him another champion will be selected if he declines. Kirk accepts the challenge thinking he can let Spock win—and only then discovers that the fight is to the death. Despite Spock's condition, he displays superior strength and agility, demonstrating his expertise with the Lirpa, a traditional Vulcan weapon. McCoy objects, telling T'Pau that Kirk is seriously disadvantaged by Vulcan's heat and thinner atmosphere (and, though it is not mentioned in the series proper, the higher gravity), and suggests he inject Kirk with a tri-ox compound to compensate. T'Pau allows the injection. The combat continues and Spock garrotes Kirk with another traditional Vulcan weapon, the Ahn'woon, at which point McCoy pronounces the Captain dead and beams back to the Enterprise with Kirk's body.
With the battle over, Spock gives up his claim to T'Pring, but questions her choice of Kirk as her champion. In a display of logic that impresses Spock, T'Pring explains that she "did not wish to be the consort of a legend," and developed a mutual attraction with Stonn. Since she could only legally divorce Spock through the koon-ut-kal-if-fee, and allowing Stonn to take the challenge as her champion would risk losing him, she instead chose Kirk, knowing that regardless of the outcome she and Stonn would be together — as Kirk would not want her, and Spock would release her for having dared to challenge. Even if Spock held her to her vows, he would return to Starfleet, "and I would have thy name, and thy property — and Stonn would still be there." She wished to be altogether free of Spock.
Spock returns to the Enterprise, but before beaming up, he informs Stonn that "After a time, you may find that having is not so pleasing a thing after all as wanting. It is not logical, but it is often true."
On the ship, Spock announces his intention to resign his commission and submit himself to Starfleet, to face the consequences for killing Kirk. However, in Sickbay he finds his Captain alive and well, and expresses overt joy in front of McCoy and Nurse Chapel. McCoy and Kirk then explain that McCoy actually injected the Captain with a neuroparalyzer drug that merely simulated death. Spock says that when he thought he had killed the Captain, he lost all desire for T'Pring. Starfleet retroactively grants permission to divert to Vulcan at T'Pau's request, thereby letting Kirk off the hook for disobeying orders. Spock denies having expressed undue emotion at seeing Kirk alive. McCoy is disgusted by this, and Kirk and Spock leave Sickbay without further illogical displays.