The crew of the interstellar ship Mercator is making valuable discoveries in the Doradoran region, but conflict is mounting between them and the inhabitants of hte Earthlike-planet Cibola. Xenologist Caty Sanjuro knows Cibolans can stop human settlements on their planet, but her superiors think she's just gone native. Sundown, a Cibolan astronomer, is working to keep the resistance peaceful, but others see no reason for caution: they have successfully fought off invaders in the past. Despite the growing friendship between Sanjuro and Sundown, preventing war is looking more and more impossible.