Two United Federation of Planets members, Elas and Troyius, suffered war for centuries. With Klingon Empire expansion approaching the planets' system, Tellun, the Federation sends the U.S.S. Enterprise to lend assistance in a peace negotiation. The peace treaty is to be completed by the bonding in marriage of the two planets' leaders, the Dohlman of Elas and the leader of Troyius. The starship provides transport for the Dohlman to Troyius, while also offering a time and place for the Dohlman to learn the duties and customs of Troyius.
Upon her arrival on the U.S.S. Enterprise, it is clear that the Dohlman, Elaan, is an arrogant and reluctant bride. Ambassador Petri from Troyius attempts to educate Elaan in preparation of her wedding, but is rewarded with a knife wound. Captain Kirk must step in as Elaan's tutor to insure that the alliance takes place. Kirk takes a strong hand in Elaan's education, but his efforts are compromised when she begins to cry and he touches one of her tears. The Elasian tears of a Dohlman carry an infectious agent which acts as a powerful aphrodisiac.
While the captain fights the effects of Elaan's tears, a Klingon battlecruiser appears. In the ensuing melee, it is apparent that the Klingon captain is attempting to lure the starship into warp speed which, as Scotty soon discovers, would destroy the ship. A saboteur from the Elasian party has damaged the dilithium crystal chamber, rendering the crystal inert, and the ship nearly powerless.
While under attack, Spock detects strong energy readings which emanate from a crystal necklace of Elaan's — made of dilithium. Fitting the raw crystals into the anti-matter chamber, the ship is able to fend off the Klingons and continue to Troyius. Kirk and McCoy discover the one true cure for the Elasian tears — prior infection — in Kirk's case, the starship Enterprise had infected him even more powerfully than had the Dohlman.