Arthur and Maudie Brightstorm are back home in Lontown and finally settling in after their exhilarating – and secret – adventures in Erythea. Maudie's sky-ak invention is a hit, and Harriet has just announced that she will share her sky-ship's hydroelectric technology. But tensions are brewing. Pitch is becoming more and more difficult to mine, and the existence of super-sapient creatures on other continents threatens how the Explorers see themselves in the natural order of the world. When Arthur and Maudie begin hearing a strange song summoning them north – a song only they can hear – they know there's an adventure waiting for them that may change the lives of everyone in the Wide. As the crew of the Aurora prepares for their trip, they're joined by new members and old friends. But they're not the only sky-ship journeying northward to uncover the region's secrets ...