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.

DS Sarah Collins is called to the brutal stabbing of a teenager, but her investigation soon finds itself at cross purposes with a covert operation led by Kieran Shaw, who is working with Sarah's old partner Steve Bradshaw, now deep undercover. Meanwhile, Lizzie Adama is back at Farlow trying to balance her new role as a detective with family life, but her path and Kieran's are about to cross.

Sarah and Elaine uncover a disturbing link between their ongoing investigation and a brutal hit-and-run. Kieran urges Lizzie not to reveal the secrets of Operation Perseus.

Kieran and Baillie share some information that could help Sarah make an arrest but her reluctance to cooperate leaves them exasperated. Shakiel puts Steve's loyalty to the test.

Operation Perseus reaches its climax as Kieran prepares to catch Shakiel with a car-load of automatic weapons. However, Spencer's killer, Jermaine King, has escaped from custody and kidnapped Ryan's sister, Tia.