The Trusts function as both business conglomerates and charitable NGOs; having restored order, their power has largely supplanted that of governments, and their managerial cadres (serving the owner-class Families) exercise quasi-feudal authority over their subordinates.

In the wider solar system, asteroid settlers that did not experience the pandemics preserve a more liberal mode of existence; they maintain cautious links with the Trusts, cooperating on matters of mutual interest.

The world is nonetheless deemed worthy of intensive investigation, as understanding an entire evolutionary process parallel to Earth's could provide important biological insights.

As firsthand research on Isis is extremely difficult, an initiative within the Trusts produces a small group of genetically engineered children specially designed to survive its environment, with enhanced immune systems capable of countering the ravenous Isian microbiome.

The sole survivor, Zoe Fisher, is rescued when her patron finally prevails in the inter-Trust power struggle, and is sent to the Isis system, where the researchers are having increasing difficulty maintaining their quarantine.

Zoe is dispatched to one of the surface stations, where she meets and befriends a young man from the asteroid-dweller contingent, and begins making unprotected exploratory trips through the Isian ecology.

Zoe's patron - having been implanted with the same experimental anti-Isian-life countermeasures before leaving Earth - is left alone on the slowly deteriorating orbital station, as the only living human in the Isis system.