London 1940. A city of darkness, a city of blackout, where every night bombs fall from the sky. Amid the devastation of the war-torn capital, some children have fallen through the gaps. Homeless, lost - they take refuge below ground, in the tube network; they survive by coming together in an abandoned station, forging out a life, in a world beneath the world. Eleven-year-old Daniel discovers this magical, subterranean city of children; he shares in its wonders and dreams, making both friends and enemies along the way. But the existence of the children is both precarious and fragile. Some of them begin to disappear. Others are found dead - laid out on the tracks. The children whisper stories about a supernatural being haunting the tunnels. The question abides: Are they alone or is there something down in the darkness with them?