In the first book in the series, Volcano in the Ocean, we met the Islanders and the Gethsemane Republic (the GR). They formed very different societies after an apocalypse, and can't understand each other. To solve their differences, they exchanged people to learn about each other. Unfortunately, the Islanders were genuinely trying to find out about the GR, but the GR were trying to find out how to invade the Islands. The Islanders who travelled to the GR are stuck in the GR, and try to go home, which presents enormous problems. The Islanders in the Islands are trying to find out ways to get the travellers home.