This book has a small mark on the back cover - SALE
 
Twelve-year old twins, Mika and Ellie, live in a future behind a wall - safe from the plague animals beyond. Or so they've been told. But when one of them disappears, and the other takes part in a sinister virtual reality game, they begin to discover their concrete world is built on lies. Determined to find each other again, they go in search of the truth. And as a strange sound in their heads grows to a roar, they find out that children and the planet have never mattered more.