A veteran beat cop and teenage girl fall to their deaths from a tower block in south-east London, leaving a five-year-old boy and rookie PC Lizzie Griffiths on the roof, only for them to go missing. DS Sarah Collins is drafted in to investigate, working to find Lizzie before she comes to serious harm, but also to uncover the truth behind the grisly tower block deaths.

Following the dramatic climax of season one, rookie PC Lizzie Adama returns to her London police station, where she finds herself fighting to protect a mother and daughter in peril. DS Sarah Collins, having transferred to Homicide Command, reopens a cold case involving a missing schoolgirl that leads her to cross paths with Lizzie again. DI Kieran Shaw wants to recruit DS Steve Bradshaw for a specialist task force targeting a major organized crime figure but must convince Steve he can be trusted.