The first episode is set two years after the previous series when Sarah is called to investigate the stabbing of a teenage boy - the case brings her into contact with Lizzie, who is now balancing life as a detective and mother, and into conflict with DI Kieran Shaw who is running Operation Perseus, an undercover police task force set up to bring down South London drug kingpin Shakiel Oliver. Kieran is working with DC Steve Bradshaw who has gone undercover to infiltrate Shakiel's empire.
The focus of Sarah and Kieran's conflict is 15-year-old Ryan Kennedy who witnessed the fatal stabbing but is also a key part of Shakiel's inner circle - as Perseus heads to its shocking climax, Ryan will have a surprising role to play.