Sarah races to Lizzie's aid and both face Brannon together in a grisly climax.

The team are left reeling when Brannon's video goes viral.

Shakiel prepares to buy a vast cache of automatic weapons and Operation Perseus moves into its final stage.

Sarah And Lizzie work together to hunt a killer to reveal secrets and possible hunters . A new suspect emerges in the Tania case.

The hit and run murder of a sex-worker appears to be linked to the stabbing, and Sarah connects this to Lizzie's investigation, putting Operation Perseus and the undercover officers at risk of discovery by king-pin Shakiel and his gang.

With Lizzie still on the run, Sarah finally persuades her boss, DCI Tim Bailie, to go public about Lizzie's disappearance.

DS Sarah Collins is tasked with a cold case. PC Lizzie Adams's willingness to do things by the book is tested.

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 Adama, who disappears within hours. Detective Sergeant Sarah Collins is charged with leading the investigation, trying to track down Lizzie before she comes to serious harm and uncovering the truth behind what happened.

A suspect in the stabbing is found but his escape jeopardises the secrecy of Operation Perseus and the safety of eyewitnesses. Meanwhile, rediscovered evidence from the Portland Tower case threatens the careers of several officers.

When Sarah finally interviews Lizzie, the explosive truth about what really happened on top of the tower emerges.