In a world full of checkpoints and controls, can love and hope defy the borders? A searing, timely story, as arresting as it is beautiful.

The world is a shell of what it once was. Now, you must prove yourself worthy of existence at every turn, at every border checkpoint. Mhairi has learnt the importance of living her own story, of speaking to no one. But then she meets a young boy with no voice at all and finds herself risking everything to take him to safety.

In this gripping near-future story, Nicky Singer tackles two urgent contemporary issues - global warming and mass migration. Desperate people are moving north, and Scotland is the new destination of dreams. Resilient 14-year-old Mhairi Bain is on her way to the Isle of Arran where her grandmother lives. Travelling alone, she has witnessed horrors that include the deaths of both parents. Tough but not heartless, she has taken pity on a mute African boy on the way.

The chapters are short and tense, written in a boiled-down style. As a way of dealing with her trauma, Mhairi has created 'Castle', a mental fortress in which to conceal bad memories. It's a neat narrative device. The pair face many obstacles, dangers and border crossings, dramatizing the plight of the migrant in a world, compellingly evoked here, that is both oddly familiar and yet horrifyingly changed.