When Spock begins to exhibit strange, erratic behavior, Kirk asks McCoy to conduct a medical examination. McCoy concludes that Spock is experiencing a potentially lethal internal distress. Spock explains to them that Vulcans are married as children with the understanding that they will fulfill this commitment when they become adults. Spock has reached this time, the "pon farr," and if he doesn't get to Vulcan immediately to mate with his bride, T'Pring, he will die.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

Celia Lovsky (T'Pau)
Arlene Martel (T'Pring)
Lawrence Montaigne (Stonn)
Majel Barrett (Nurse Christine Chapel)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Walter Koenig (Ensign Pavel Chekov)
Byron Morrow (Admiral Komack)

• The Vulcan T'Pau is to officiate Spock's wedding. She is the only person to turn down a seat on the Federation council.
  
• Instead of marrying Spock, T'Pring chooses the kal-if-fee (the challenge) and chooses Kirk to fight Spock to the death. Having defeated Kirk in the challenge, Spock's pon farr is resolved and Spock allows T'Pring to wed Stonn.
  
• This episode marks the first use of the Vulcan hand gesture and the phrase "Live long and prosper."
  
• This is the first appearance of Ensign Pavel Chekov, the new navigator.
  
• Chief Engineer Lt. Cmdr. Scott does not appear in this episode, chosen to open Season 2.
  
• DeForest Kelley (Dr. McCoy) becomes a regular cast member starting in Season 2 and his name now appears in the opening credits after Leonard Nimoy.
  
• The theme song has added a female vocalizing the melody. The end credits version also has a slightly different instrumental arrangement.