T'Pring
Arlene Martel

T'Pau
Celia Lovsky

Stonn
Lawrence Montaigne

Admiral Komack
Byron Morrow

With the Enterprise currently en route to Altair VI, McCoy takes Captain James T. Kirk to one side and informs him that Spock has been behaving strangely recently, not eating and seeming almost 'nervous'. As if to confirm the fact, the door to Spock's quarters opens and Nurse Christine Chapel is thrown out by Spock who is annoyed that she has taken him some of his favourite plomik soup. He goes to Kirk and requests an immediate leave of absence on Vulcan but avoids Kirk's concerned enquiries and refuses to give a reason.

Despite their current mission, Kirk changes course for Vulcan but soon receives orders from Admiral Komack at Starfleet to speed up their arrival at Altair VI, which makes the detour impossible. During the night, Spock alters the ship's course back to Vulcan but, on being questioned by Kirk, denies all knowledge of it. He realises that he must have done it and asks to be taken into custody saying that no Vulcan could discuss what is happening to him.

McCoy eventually persuades the unwilling Vulcan to undergo a medical examination after which McCoy informs Kirk that if he doesn't get Spock to Vulcan within eight days, Spock will die due to the metabolic changes occurring inside him for no apparent reason. Kirk demands an explanation from Spock who is eventually persuaded by their friendship to tell him that he is gripped with pon farr, the Vulcan need to mate that Vulcans undergo every seven years. This causes a loss of emotional control and, if he cannot get to Vulcan to mate, he will die. Kirk swears himself to secrecy and tries to persuade Starfleet to allow the detour but is refused, forcing him to choose between friendship and duty.

The Enterprise changes course and heads towards Vulcan at maximum warp. During the trip, Spock requests McCoy and Kirk, being his closest friends, to accompany him to the Vulcan ritual called koon-ut-kal-if-ee which is to be held on his family's ancestral land. On arrival, they beam down to Vulcan where Spock explains that he and his 'bride', called T'Pring, were psychically joined when they were children. As the ceremonial party approaches Kirk is amazed to see that the head of the ceremony is T'Pau, the only person ever to turn down a place on the Federation Council.

Spock introduces them and she explains that what they will observe has been unchanged since the start of Vulcan civilisation. The ritual begins, but is suddenly stopped by T'Pau as the 'right of challenge' has been instigated. By now, Spock is in the throes of plak-tow, the blood fever, and T'Pau explains that Spock will have to fight for T'Pring, who has the right to name her champion. McCoy is stunned in amazement as she chooses James T. Kirk. A Vulcan called Stonn complains that the honour was to be his but T'Pau insists that the choice has been made although, because they are off-worlders, she gives Kirk the opportunity to decline, in which case another champion will be chosen. Kirk, realising that Stonn would probably beat Spock in his present condition, accepts the challenge and only then finds out that it is to the death!

During a break in the fight McCoy gives Kirk an injection, telling him that it is a tri-ox compound to compensate for the thin atmosphere. When the contest recommences, Spock eventually subdues Kirk, dropping his lifeless body to the ground. In victory he recovers his senses and realises what has happened, but McCoy has already pronounced Kirk dead and had his body transported back aboard the Enterprise. On being queried by Spock, T'Pring tells him that she did not want to be the wife of a Vulcan 'legend', but did want Stonn. If Kirk won, he would not want her and if Spock won, neither would he because she had challenged him. Spock congratulates her on her logic and frees her from her vows.

He takes his leave of T'Pau where, in answer to her ritual Vulcan 'live long and prosper' he replies, 'I shall do neither. I have killed my captain and my friend'. Having already ordered the Enterprise to set course for the nearest Starbase, he returns to the ship so that he can surrender himself to the authorities as soon as possible. He can hardly control himself and lets his emotions show fleetingly as he is greeted back on board by an obviously alive Captain Kirk. Apparently, McCoy had injected him with a neural anaesthetic that made him appear to be dead. Still shaken, he attempts to make an implausible excuse for his 'emotional outburst' as concern for Starfleet.