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.

With Lizzie still on the run, Sarah finally persuades her boss, DCI Tim Bailie, to go public about Lizzie's disappearance. Later, she makes a series of discoveries about the events that led up to the Portland Tower fall which further complicate the case.

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

Sarah searches for a vanished teenager; Lizzie investigates a case of domestic violence.

Sarah and Lizzie find themselves working on the same murder case.

As the manhunt closes in on Brannon, Sarah realizes where Tania's body might be buried.

The Brannon manhunt reaches its climax, and Sarah faces Tania's killer.