I'll admit upfront this is a nostalgic favorite. I think it's also very good.
Early in her career, Lt. Uhura met a young diplomat from the world of Eeiauo. The two women bonded over music, singing, and the songs of their respective peoples and cultures. The graceful, catlike Sunfall of Ennian even shares songs with Uhura, and the Old Tongue they are sung in, that are not to shared except with other bards,
Years later, Uhura is now communications officer on the USS Enterprise, which is on a mission of mercy to Eeiauo, where a terrible plague has broken out. The infected individuals become weak, their fur (or hair, as they make the unhappy discovery it also affects humans), become stiff and achy, fall into comas, and die. One of those dying is Uhura's old friend, Sunfall.
