The Enterprise transports Elaan, a member of the ruling dynasty of the warrior Elasians, to the planet of their enemy, the Troyians, so that her arranged marriage will halt their interplanetary war. Kirk must teach the arrogant, vicious princess the ways of polite society but falls victim to the legendary weapon of Elasian women: their tears serve as a love potion. Spock tracks what may be another ship shadowing the Enterprise and Kirk, distracted by Elaan, must fight against other parties interested in preventing the Federation's peace efforts.