Sparrow is cold, hungry and alone, trudging through the grey streets of London. He spends his days looking hopefully for food and shelter, and his nights huddled miserably in dark alleys, shivering in the night air. He dreams of a better life, somewhere warm and dry, with lots of friends for company and plenty of food for all. His life changes when he meets Jimmy, who shows him a new life underground. A group of gifted children have created a society of their own in an abandoned Tube station, away from the problems in the world above. Soon Sparrow discovers he has amazing powers of his own, abilities he never knew he had. Sparrow and Jimmy explore the network of tunnels under London, as they try to solve a mysterious riddle and find a long-lost treasure. Sparrow has little time to master his new powers, if he is to defeat the many foes awaiting him. Meanwhile unfriendly eyes are following Sparrow as he adapts to his new life. Someone is watching his every move, someone powerful who has plans of his own.