A dynastic marriage between Elaan, the Dohlman of Elas, and the ruler of Troyius, arranged to end the hostility between the two worlds of the Tellun system and escorted by the Enterprise.
In the Original Series episode Elaan of Troyius, the Enterprise is ordered to carry Elaan from Elas to her wedding on Troyius at slow speed, giving the Troyian ambassador Petri time to teach her his people's manners. She despises the match and stabs Petri, leaving Kirk to take over the lessons.
The tears of Elasian women act as a powerful love drug, and Kirk is affected when he touches hers. At the same time a Klingon warship shadows the Enterprise, and one of Elaan's guards, Kryton, sabotages the engines on the Klingons' behalf so that the ship cannot fight or flee.
The Klingon interest is explained when Spock finds that Elaan's necklace is made of raw dilithium, common in the system. Scott uses the stones to restore power, the Klingons are driven off, and Elaan goes to her wedding in the end out of duty.