DS Sarah Collins investigates the deaths of a long-serving police officer and a 15-year-old Libyan refugee in southeast London.
Sarah finally persuades her boss DCI Tim Bailie to go public about Lizzie's disappearance, but she then makes a series of discoveries that complicate the case.
Lizzie decides to turn herself in, and the explosive truth about what really happened on top of the tower emerges as Sarah finally gets a chance to interview her.
DS Sarah Collins joins Homicide Command and is tasked with a cold case, and PC Lizzie Adama's willingness to do things by the book is tested by a call-out.
Sarah and Lizzie find themselves working the same murder with their mutual distrust still apparent, but they must cooperate if they are to find a lost child.
The team are left reeling when Brannon's YouTube video goes viral, and Sarah and Elaine make a grim discovery that exposes a vile predator and a vital clue.
When Steve alerts Sarah to the danger awaiting Lizzie at her home, Sarah races to her aid and they face the desperate Brannon together in a grisly climax.
A stabbing leads DS Sarah Collins into the path of an undercover unit led by Kieran Shaw, and Lizzie Adama is back at Farlow balancing being a DC with family life.
Sarah uncovers a disturbing link in her ongoing investigation into working girl Lexi's death as Kieran urges Lizzie not to reveal the secrets of Operation Perseus.
Sarah's reluctance to cooperate leaves Kieran and Bailie exasperated, and Steve is under the spotlight as Shakiel puts his loyalty to the test.
The culmination of years of painstaking investigation and surveillance is upon Kieran, Steve and Operation Perseus as they prepare to for the final push.
