Spock's pon farr forces the Enterprise to Vulcan, where he must fight Kirk or die.
Spock begins to behave erratically, and McCoy discovers that he is entering pon farr, a Vulcan mating cycle that will kill him unless he returns to Vulcan. Kirk defies orders to take him home. There the ritual of marriage leads to a challenge, and Kirk is forced to fight to the death.
The season-opening episode gave the first views of Vulcan culture, the Vulcan salute, the greeting about living long and prospering, and T'Pring. Its music, written by Gerald Fried, was reused many times. Fans love it for deepening Spock and for its brilliant final scene.