Stardate 3372.7: As the Enterprise is en route, Kirk meets with McCoy who mentions that Spock has been acting… strangely. The First Officer is avoiding food and threatened to break Dr. McCoy's neck. Nurse Chapel takes Vulcan plomik soup to Spock but he throws her out and tosses the soup after her. He then asks Kirk to grant him a leave of absence to Vulcan.
Later Kirk demand an answer from Spock about his request for shore leave and tries to suggest an alternate locale, but Spock insists it must be Vulcan. Kirk relents but later receives orders to take the Enterprise directly to Altair 6 for an inauguration ceremony when its schedule is advanced. Spock accepts the situation, but later when Kirk checks in he discovers that Spock has given orders to proceed to Vulcan. When Kirk confronts his first officer, Spock says he has no memory of giving such orders and the captain sends him to sickbay.
McCoy meets with Kirk and tells him Spock must get to Vulcan within 7-8 days, he'll die. The doctor can't account for it but Spock has a growing bodily imbalance, but Spock won't explain it. Kirk goes to see Spock, who is reviewing a picture of a young Vulcan girl. Kirk demands answers and promises to keep it confidential. Spock admits that it is a matter of Vulcan reproduction, and they use a ritual that brings madness which rips away the Vulcan logic and civilized behavior. Every seven years a Vulcan must participate in the ritual of Pon Farr or die trying. Kirk agrees and goes to his superior, Admiral Komack, to let him know he's diverting to Vulcan. Komack refuses to grant him permission so Kirk resolves to go there anyway. Upon arrival, Spock asks Kirk and McCoy to stand with him at the ceremony. They are greeted by Vulcan Space Command and Spock's fiancé, T'Pring, greets him.
Kirk, Spock, McCoy beam down and arrive at the ceremonial circle. Spock sounds the gong for the marriage party and explains that his mind and T'Pring had their minds bonded together when they were young. The marriage party arrives, led by T'Pau, the ranking Vulcan matriarch. Spock makes introductions and then prepares to complete the ritual, but T'Pring interrupts. Spock goes into a trance while T'Pau explains that T'Pring has offered challenge and must choose a champion. Spock is in plak tow, the blood madness, and isn't thinking rationally. Kirk and McCoy insist on staying and T'Pring confirms she will become the property of the victor. Spock accepts challenge and T'Pring chooses her champion… Kirk.
T'Pring's paramour, Stonn, objects, and T'Pau acknowledges that Kirk as an outsider doesn't have to accept the challenge. Holding on to his rationality, Spock tries to ask T'Pau to release Kirk but she refuses. Kirk figures that he can knock out Spock and finally accepts the challenge over McCoy's warning that Vulcan's heat and thin air will hinder him, and will quit if he gets in over his head. Kirk accepts but soon realizes that the battle is to the death… and no forfeit is accepted.
The fevered Spock fights and Kirk is hard put to defend himself. They fight until both are disarmed and McCoy asks permission to give Kirk a shot to compensate for the air. The battle then continues with leather strap and Spock garrotes Kirk to death. McCoy declares the captain dead and beams up. Spock, now free of the blood madness, demands an answer from T'Pring. She explains that she was in love with Stonn and she issued the challenge concluding that whatever happened, she would end up with Stonn. Spock releases her from her vows and beams up to the ship.
Once aboard, Spock prepares to resign his command when Kirk walks in. Spock has a brief highly emotional reaction, and Kirk explains that McCoy gave him a shot that simulated death. They are told that T'Pau requested the Enterprise's diversion and any delay is okay. They proceed for Altair 6 but first McCoy has to point out that Spock had an emotional reaction. Spock tries to explain his reaction logically, but Kirk and McCoy aren't buying it for a minute.