Journey to Vulcan

On Stardate 3372.7, in the year 2267, Mr. Spock, first officer of the Constitution-class Federation starship USS Enterprise NCC-1701, abruptly requests a leave of absence to his home planet of Vulcan after displaying irrational behavior for several days. Captain James T. Kirk and Dr. Leonard McCoy witness one of Spock's outbursts, including the throwing of a bowl of Vulcan plomeek soup specially prepared for him by Nurse Chapel, and McCoy agrees Spock needs some "time off".
Kirk is baffled by the strange behavior and Spock's unwillingness to tell him more than "I need rest", but honors Spock's request and 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. Later, Kirk asks Mr. Chekov how long it would take to get to Altair if they diverted to Vulcan just long enough to drop off Mr. Spock; Chekov replies that Spock has already ordered a course change to Vulcan.
Kirk confronts Spock, who says he has no memory of having changed the order. Kirk orders him to report to sickbay for a full examination. Once there, 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 from the physical and psychological stress. Spock does not wish to discuss what is currently affecting him, but Kirk demands an explanation.
Seemingly embarrassed, Spock informs that his condition is called Pon farr, a very personal biological syndrome that all Vulcan males painfully endure periodically throughout their adult life. During this time, they must join with their wives, or die. Spock compares his need to return to Vulcan to the need for eel birds of Regulus to return every eleven years to the caverns where they hatched, and also to salmon of Earth who must return to the stream where they were born to spawn. Kirk gets the idea that it is essentially Spock's mating season.
Kirk contacts Admiral Komack at Starfleet Command Sector 9, 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 Altair VI ceremony. Spock seems to calm down once he learns Kirk is doing what he can.
The Enterprise finally arrives at Vulcan, and Spock invites Kirk and Dr. McCoy to accompany him to the surface. Once there, Kirk comments on Vulcan's extremely harsh environment and its atmosphere which is thinner than Earth's.
Spock explains to them that Vulcans are married as children 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, awaits him. He must enter Kunat kalifee, his family's traditional place of the Vulcan marriage ceremony.
T'Pau, a highly respected member of Vulcan society, and best known as the only person to ever turn down a seat in 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 kal-if-fee, her customary right to a physical challenge between Spock and Stonn. But instead, she picks Kirk to be her challenger to fight Spock. The duel pains Spock, and he asks that T'Pau forbid it because Kirk "does not understand, he does not know", but T'Pau allows it. She tells Kirk to decide, telling him another champion will be selected if he declines. Kirk accepts the challenge, thinking he can let Spock win — then discovers that this is a fight to the death.
Regardless of Spock's condition, he displays superior strength and agility. Kirk is weakened by Vulcan's heat and thinner atmosphere and must struggle harder against Spock's strength. McCoy objects, telling T'Pau that Kirk is seriously disadvantaged, 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 a traditional weapon, at which point McCoy pronounces the Captain dead and has his body beamed back to the Enterprise.
With the battle over, Spock now realizes what has happened. He gives up his claim on T'Pring and sadly returns to the ship, though not before questioning T'Pring on her choice of Kirk as her champion; in a display of logic that impresses Spock in its flawlessness, T'Pring explains that with Spock's name now near-legendary among their people, she developed a mutual attraction with Stonn rather than becoming the consort to such fame. However, since she could only legally divorce Spock through the kalifee, 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 Spock would either be dead or in prison for killing Kirk.
On the ship, Spock announces his intention to resign from Starfleet and submit himself to whatever consequences await him for killing Kirk. However, he finds his Captain alive and well, and expresses overt joy (unusual for a normally emotionless Vulcan) in the full witness of McCoy and Chapel. McCoy and Kirk then explain that McCoy actually injected the Captain with a neuroparalyzer drug that simulated death but merely knocked him out. Spock says that when he thought he had killed the Captain, he found that he had lost all desire for T'Pring, and the madness was gone. Furthermore, Kirk is let off the hook for disobeying orders when Starfleet retroactively grants permission to divert to Vulcan at T'Pau's request.