Twenty-five years into the future, no humans are allowed in to the Wildlands – a vast area in Britain where wolves, lynx and bison roam free. The only exception is a high-speed train line between London and Glasgow that crosses right through the heart of the project.

Thirteen-year-old Astrid and her little sister, Indie, are onboard when their train slows to a brief, unexpected stop ... and they find themselves accidentally left behind. Stranded in this place of astonishing natural wonders and terrible dangers, they have only a rucksack, a phone without signal – and each other.