DS Sarah Collins and her colleague, DC Steve Bradshaw, officers in the Directorate of Special Investigations (DSI), are called to an incident at Portland Tower in south east London. They find at the bottom of the tower the dead bodies of long-serving police officer, PC Hadley Matthews, and a 15-year-old Libyan refugee, Farah Mehenni. Up on the roof Sarah finds a traumatised five-year-old boy and a rookie police officer, PC Lizzie Adama – the only witnesses to what happened.

Lizzie decides to turn herself in, but first she meets Kieran Shaw in secret, who tries to persuade her not to implicate herself. When Sarah finally interviews Lizzie, the explosive truth about what really happened on top of the tower emerges.