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. Upon her arrival on the U.S.S. Enterprise, it is clear that Dohlman Elaan of Elas is an arrogant and reluctant bride. After stabbing the Troyius ambassador, Kirk must step in as Elaan's tutor to insure that the alliance takes place. His efforts are compromised when she begins to cry and he touches one of her tears, which carry an infectious agent that acts as a powerful aphrodisiac. While the captain fights the effects of Elaan's tears, a Klingon battlecruiser appears.

James Doohan (Lt. Cmdr. Montgomery Scott)
France Nuyen (Elaan)
Jay Robinson (Lord Petri)
Tony Young (Kryton)
Nichelle Nichols (Lt. Uhura)
George Takei (Lt. Hikaru Sulu)
Walter Koenig (Ensign Pavel Chekov)
Majel Barrett (Nurse Christine Chapel)
Lee Duncan (Lt. Evans)
Dick Durock (Elasian Guard #1)
Charles Beck (Elasian Guard #2)
K.L. Smith (Klingon)
Victor Brandt (Technician Watson)