A veteran beat cop and a teenage girl fall to their deaths from a tower block in south-east London. Left alive on the roof are a five-year-old boy and rookie police officer Lizzie Griffiths. Within hours, Lizzie has disappeared, and Detective Sergeant Sarah Collins is charged with leading the investigation. She not only has to track down Lizzie before she comes to serious harm, but also to uncover the truth behind the grisly deaths. What chain of events took two police officers and two children to the roof of that tower block - why did two of them die? Why did two of them survive?