The planet Eeiauo is threatened by an epidemic, which crosses over to humans and starts spreading through the Federation. Captain Kirk and the Enterprise (along with a stand-in medical officer, Dr Evan Wilson, as McCoy is helping deal with the situation on Eeiauo) set out to find a planet that may hold a cure. Their only clue to the planet's location is a folk song Uhura learned from an Eeiauoan friend years ago.
This novel contains examples of:
- Alien Non-Interference Clause: The usual Star Trek non-interference directive is mentioned. In fact, the Federation Council chooses to grant an exemption to it so the Enterprise can visit a planet occupied by a pre-contact society — a sign of how serious the threat they're facing is.
- Blazing Inferno Hellfire Sauce: The Sivaoans use the berry of a native bush as a spice. They call it a "tail-kinker" and a favorite prank is to slip a few of the fresh berries into a dish rather than the processed version. Think getting something you think contains a bit of red chili flakes but instead has whole Sichuan peppers in it.
- Cat Folk: The Eeiauoans and their relatives the Sivaoans.
- Fascinating Eyebrow: Spock's eyebrow technique is lampshaded — "Eyebrow on stun, Mr. Spock!"
- Godzilla Threshold: It's a sign of how serious the situation is that the Federation Council gives the Enterprise permission to break the Prime Directive if Kirk judges it necessary to do so.
- No Biochemical Barriers: The Federation is threatened by an epidemic that starts with the catlike Eeiauoans but crosses into other species including humans.
- Photographic Memory: The Sivaoans have this kind of memory. It does turn out to have some shortcomings, one of which is a major plot point.
- Schizo Tech: The Sivaoans. Turns out they were Luddite aesthetics that make up for their lack of a high tech infrastructure with photographic memories, and the related Eeiauoans were a technology-embracing splinter group that was exiled from Sivao after a cure was found for the disease that later becomes an epidemic. This was because the future Eeiauoans lived in cities, which allowed diseases to spread, and the resulting epidemics led to the extinction of several plants and animals.