Spock Amok

The crew go on shore leave while the Enterprise undergoes repairs at Starbase One. Number One and Noonien-Singh apprehend two ensigns conducting an unauthorized spacewalk as part of a game played by lower-ranked crew members called "Enterprise Bingo". Learning that they have reputations as "fun-killers", the pair try the game themselves. The half-human Spock has grown concerned that T'Pring thinks he is becoming too human. He plans to spend his shore leave with her but April interrupts to ask Pike and Spock for help negotiating with the R'ongovians, who are considering allying with the Federation, Klingons, or Romulans. After discussing his relationship with Chapel, Spock undertakes a special mind meld ceremony with T'Pring so they can understand each other better. This accidentally causes them to switch katras, effectively swapping bodies.

Summary from Wikipedia, CC BY-SA 4.0.