This installment of The Original Series showcases First Officer Spock journeying back to his home planet for a savage Vulcan nuptial ceremony. It is the exclusive episode of the show to display scenes on Vulcan.

It is the first episode to air with Ensign Pavel Chekov (Walter Koenig) as the spacecraft's navigator and the first episode to credit DeForest Kelley as Dr. McCoy in the opening credits. It was also the initial episode broadcast in the series' new time slot of 8:30 pm on Friday night. Moreover, this is the first installment to feature the "Vulcan salute" and to present the notion of pon farr.

Spock, the first officer of the USS Enterprise, suddenly begins acting oddly and requests to be given leave to visit his home planet of Vulcan. Knowing something is wrong, Captain Kirk and Chief Medical Officer Dr. McCoy permit him to go and Captain Kirk re-directs the ship to Vulcan. However, before they can get there, Kirk gets orders from Starfleet to take the Enterprise to Altair VI for the inauguration ceremony of their new president. Kirk orders the crew to change the ship's course to Altair VI, yet Spock secretly alters the route back to Vulcan. When Kirk discovers this, he orders Spock to Sick Bay, where McCoy discovers that Spock suffers from a severe physical. Emotional trauma, known as pon farr, is a condition Vulcans experience at certain times in their lives. Without treatment, Spock will die within eight days. He explains that he must mate or die.

Kirk reaches out to Starfleet to request that he deviate from the mission and go to Vulcan, but his plea is declined. Nonetheless, he goes against the orders as he thinks saving his close one's life is more important than his career.

Spock invites Kirk and McCoy to attend the wedding ceremony when they arrive at Vulcan. He explains that Vulcans are betrothed as kids to fulfill the pon farr obligation and that T'Pring is to be his soulmate. T'Pring shows up with Stonn, a pure Vulcan, whom she favors over Spock. T'Pau, the only person who has ever been denied a Federation Council seat, is ready to preside over the ceremony. But, to everyone's shock, T'Pring demands the Kal-if-fee, a physical combat between Spock and her chosen champion. She surprisingly picks Kirk instead of Stonn. Spock requests T'Pau to forbid it as Kirk is unaware of the consequences, but T'Pau leaves the choice for Kirk; if he declines, another combatant will be elected. Kirk accepts the challenge, only to discover that it is "to the death."

Kirk and Spock engage in a duel with lirpa, a customary Vulcan armament. Even though Spock is weakened, he still manages to rival Kirk with his strength, agility, and Vulcan's thinner air. After McCoy speaks to T'Pau, he is allowed to inject Kirk with a tri-ox compound to neutralize the effects of Vulcan's atmosphere. The fight continues, and Spock finishes off Kirk with an ahn'woon. McCoy runs to Kirk's body, announcing his death and urgently asking to be transported back to the Enterprise.

Spock questioned T'Pring for her reasons for withdrawing his suit, and she replied that the fear of losing Stonn in the Kal-if-fee was the cause. She had calculated that if Spock were to triumph, he would release her from the marriage; if Kirk won, he would have no interest in her. Now safe from the pon farr, Spock praised T'Pring's perfect logic and departed from the Enterprise, cautioning Stonn that "the joy of possessing is not as great as the anticipation of desiring."

When Spock is on the vessel, he declares his motive to step down from his rank and face the consequences for killing Kirk, but McCoy reveals that Kirk is still alive, only in a state of paralysis due to a unique injection to him. Spock explains that his enthusiasm for T'Pring evaporated once he was convinced Kirk was dead. Kirk finds that, at T'Pau's bidding, Starfleet has finally allowed the Enterprise to go to Vulcan.