A fatal street stabbing leads Sarah Collins towards an undercover operation by her colleague who fear a massive escalation in gang violence. Lizzie is given greater responsibility at work, putting pressure on her at home as a new mother.
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.
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.