Elaan of Troyius

The Enterprise is sent to carry Elaan, the Dohlman of Elas, to the rival world of Troyius, where she is to marry its ruler and bring peace to the warring planets. Elaan is arrogant and violent, and when she stabs Ambassador Petri, who was meant to teach her Troyian manners, Kirk takes over the lessons himself. When he comforts her, her tears, which hold a chemical that binds men to Elasian women, leave him helplessly in love. Meanwhile Kryton, the jealous head of her guard, sabotages the warp drive and signals a Klingon battle cruiser before taking his own life. The Klingons want the dilithium crystals hidden in Elaan's royal necklace. Using those crystals, Scotty and Spock restore power and the Enterprise defeats the attacker. Kirk puts duty first and delivers Elaan to her wedding. McCoy finds an antidote, but Spock observes that the captain's true love has always been his ship.