A plague has been ravaging the planet Eeiauo, a world of cat-like beings, and the Enterprise has come to try to contain it before it kills all the Eeiauoans or spreads to other planets. Years earlier, Uhura had befriended a diplomat from Eeiauo, and the two women had exchanged songs as an intimate bond, swearing never to reveal them to anyone. But the only hope for a cure may lie in the song given to Uhura, and they must penetrate its layers of mystery if there's any hope for Eeiauo, or the Enterprise.